U.S. patent application number 13/127744 was filed with the patent office on 2011-12-01 for catalyst precursors, catalysts and methods of producing same.
This patent application is currently assigned to Institut National De La Recherche Scientifique. Invention is credited to Jean-Pol Dodelet, Allan S. Hay, Frederic Jaouen, Michel Lefevre, Eric Proietti.
Application Number | 20110294658 13/127744 |
Document ID | / |
Family ID | 42152419 |
Filed Date | 2011-12-01 |
United States Patent
Application |
20110294658 |
Kind Code |
A1 |
Lefevre; Michel ; et
al. |
December 1, 2011 |
CATALYST PRECURSORS, CATALYSTS AND METHODS OF PRODUCING SAME
Abstract
A catalyst precursor comprising (A) a microporous support, (B) a
non-noble metal precursor, and (C) a pore-filler, wherein the
micropores of the microporous support are filled with the
pore-filler and the non-noble metal precursor so that the micropore
surface area of the catalyst precursor is substantially smaller
than the micropore surface area of the support when the pore-filler
and the non-noble metal precursor are absent is provided. Also, a
catalyst comprising the above catalyst precursor, wherein the
catalyst precursor has been pyrolysed so that the micropore surface
area of the catalyst is substantially larger than the micropore
surface area of catalyst precursor, with the proviso that the
pyrolysis is performed in the presence of a gas that is a nitrogen
precursor when the microporous support, the non-noble metal
precursor and the pore-filler are not nitrogen precursors is also
provided. Methods of producing the catalyst precursor and the
catalyst are provided.
Inventors: |
Lefevre; Michel;
(Sainte-Julie, CA) ; Proietti; Eric; (Montreal,
CA) ; Jaouen; Frederic; (Montpellier, FR) ;
Dodelet; Jean-Pol; (Sainte-Julie, CA) ; Hay; Allan
S.; (Montreal, CA) |
Assignee: |
Institut National De La Recherche
Scientifique
Quebec
CA
|
Family ID: |
42152419 |
Appl. No.: |
13/127744 |
Filed: |
October 2, 2009 |
PCT Filed: |
October 2, 2009 |
PCT NO: |
PCT/CA2009/001365 |
371 Date: |
July 19, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61112844 |
Nov 10, 2008 |
|
|
|
Current U.S.
Class: |
502/185 ;
502/200; 502/300 |
Current CPC
Class: |
B01J 21/18 20130101;
Y02E 60/50 20130101; B01J 35/1028 20130101; B01J 37/084 20130101;
B01J 23/745 20130101; H01M 4/90 20130101; B01J 37/0018 20130101;
H01M 4/9083 20130101; B01J 23/75 20130101; H01M 2008/1095
20130101 |
Class at
Publication: |
502/185 ;
502/300; 502/200 |
International
Class: |
B01J 35/10 20060101
B01J035/10; B01J 27/24 20060101 B01J027/24; B01J 21/18 20060101
B01J021/18 |
Claims
1. A catalyst precursor comprising: a. a microporous support; b. a
non-noble metal precursor; and c. a pore-filler, wherein the
micropores of the microporous support are filled with the
pore-filler and the non-noble metal precursor so that the micropore
surface area of the catalyst precursor is substantially smaller
than the micropore surface area of the support when the pore-filler
and the non-noble metal precursor are absent.
2. The catalyst precursor of claim 1, wherein at least one of the
microporous support, the non-noble metal precursor or the
pore-filler is a nitrogen precursor.
3. The catalyst precursor of claim 1, wherein the microporous
support is highly microporous and/or carbon-based.
4. (canceled)
5. (canceled)
6. The catalyst precursor of claim 1, wherein the non-noble metal
precursor is a precursor of iron or cobalt.
7. (canceled)
8. The catalyst precursor of claim 6 having an iron loading of
about 0.2 wt % or more based on the total weight of the catalyst
precursor.
9. (canceled)
10. The catalyst precursor of claim 1, wherein the non-noble metal
precursor is a salt of a non-noble metal or an organometallic
complex of a non-noble metal.
11. (canceled)
12. The catalyst precursor of claim 1, wherein the non-noble metal
precursor and the pore-filler are the same molecule.
13. The catalyst precursor of claim 1, wherein the pore-filler
comprises a polycyclic structure.
14. (canceled)
15. The catalyst precursor of claim 1, wherein the micropore
surface area of the catalyst precursor is at most about 75% of the
micropore surface area of the support when the pore-filler and the
non-noble metal precursor are absent.
16. (canceled)
17. The catalyst precursor of claim 1, wherein a
pore-filler/microporous support mass ratio is 50:50.
18. A catalyst comprising the catalyst precursor of claim 1,
wherein said catalyst precursor has been pyrolysed so that the
micropore surface area of the catalyst is substantially larger than
the micropore surface area of catalyst precursor, with the proviso
that the pyrolysis is performed in the presence of a gas that is a
nitrogen precursor when the microporous support, the non-noble
metal precursor and the pore-filler are not nitrogen
precursors.
19. The catalyst of claim 18, wherein the micropore surface area of
the catalyst is at least about 50% of the micropore surface area of
the support when the pore-filler and the non-noble metal precursor
are absent.
20. (canceled)
21. The catalyst of claim 18, wherein a mass loss during pyrolysis
is about equal to a pore-filler loading in the catalyst
precursor.
22. The catalyst of claim 18, having a nitrogen content of about
0.5 wt % or more based on the total weight of the catalyst.
23. (canceled)
24. (canceled)
25. A method for producing a catalyst precursor, the method
comprising: a. providing a microporous support; a non-noble metal
precursor; and a pore-filler; and b. filling the micropores of the
microporous support with the pore-filler and the non-noble metal
precursor so that the micropore surface area of the catalyst
precursor is substantially smaller than the micropore surface area
of the support when the pore-filler and the non-noble metal
precursor are absent.
26. The method of claim 25, wherein the micropores of the
microporous support are filled with the pore-filler and the
non-noble metal precursor by ballmilling or resonant acoustic
mixing with or without a grinding medium.
27. (canceled)
28. (canceled)
29. A method of producing a catalyst, the method comprising a.
providing a catalyst precursor comprising a microporous support; a
non-noble metal precursor; and a pore-filler, wherein the
micropores of the microporous support are filled with the
pore-filler and the non-noble metal precursor so that the micropore
surface area of the catalyst precursor is substantially smaller
than the micropore surface area of the support when the pore-filler
and the non-noble metal precursor are absent; and b. pyrolyzing
said catalyst precursor so that the micropore surface area of the
catalyst is substantially larger than the micropore surface area of
catalyst precursor, with the proviso that the pyrolysis is
performed in the presence of a gas that is a nitrogen precursor
when the microporous support, the non-noble metal precursor and the
pore-filler are not nitrogen precursors.
30. The method of claim 29, wherein the pyrolysis is performed in a
nitrogen-containing reactive gas or vapor.
31. (canceled)
32. The method of claim 29, wherein the pyrolysis is performed in
an inert gas.
33. (canceled)
34. The method of claim 32, wherein a second pyrolysis in a
nitrogen-containing reactive gas or vapor is performed following
the pyrolysis performed in the inert gas.
35. (canceled)
36. (canceled)
37. (canceled)
38. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This PCT application claims priority on U.S. provisional
application Ser. No. 61/112,844 filed on Nov. 10, 2008.
FIELD OF THE INVENTION
[0002] The present invention relates to catalyst precursors,
catalysts, and methods of producing these catalyst precursors and
catalysts. More specifically, the present invention is concerned
with non-noble metal catalysts. Such materials can be used in
oxygen reduction reactions in fuel cells, including acid or
alkaline polymer electrolyte membrane fuel cells and metal-air
batteries.
BACKGROUND OF THE INVENTION
[0003] Clean, efficient and versatile, H.sub.2--O.sub.2 (air)
polymer electrolyte membrane fuel cells (PEMFCs) are seen as worthy
alternatives to a wide range of conventional power generation
devices such as internal combustion engines, batteries and
diesel-fuelled back-up power systems. PEMFCs generate electricity
via two electrochemical reactions that involve the oxidation of
hydrogen at the anode (2H.sub.2.fwdarw.4H.sup.++4e.sup.-) and the
reduction of oxygen at the cathode
(O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O), thus producing only
water and heat. Due to the rather low operating temperature of
PEMFCs (ca. 80.degree. C.), catalysts play an essential role in
boosting the reaction kinetics to produce the desired high power
densities.
[0004] Today, the only viable electrocatalysts used in PEMFCs are
platinum-based. Platinum is considered to be a "noble" metal, such
as gold, for example. In PEMFCs, 90% of the platinum is needed at
the cathode due to the sluggishness of the oxygen reduction
reaction (ORR) compared to the fast hydrogen oxidation reaction at
the anode. Despite improved platinum performances, the increasingly
prohibitive cost of platinum remains a major obstacle for the
commercial viability of PEMFCs.
[0005] In addition, the production of platinum in the natural world
is rather limited. On the other hand, there are estimations that
the demand for platinum as electrode catalyst will increase
significantly as the demand for electric cars with fuel cells
increases. Therefore, there is a fear of a still further rise in
platinum prices. Accordingly, electrode catalysts which can be
formed without using noble metals, such as platinum, are
desirable.
[0006] Research activity into non-noble or non-precious metal
catalysts (NPMC) for the ORR has grown considerably since 1964 when
Jasinski observed that cobalt phthalocyanine catalyzed the ORR
(REFERENCE 1). Such catalysts were first obtained by adsorbing
Fe--N.sub.4 or Co--N.sub.4 macrocycles on a carbon support and
pyrolysing the resulting material in an inert atmosphere (REFERENCE
2). Since, NPMC research using metal-N.sub.4 macrocycles has
continued (REFERENCE 3-5).
[0007] A breakthrough was then achieved when it was revealed that
these often-expensive macrocycles could, instead, be substituted by
individual N and Co precursors (REFERENCE 6). This approach was
followed by several groups (REFERENCE 2, 7, 8-17).
[0008] One approach in the synthesis of NPMCs for ORR has been to
use NH.sub.3 as a nitrogen precursor. The catalysts are obtained by
wet impregnation of a carbon black with an iron precursor like
irons.sup.II acetate (FeAc), followed by a heat treatment, i.e.
pyrolysis, in NH.sub.3. Herein, such electrocatalysts will be
referred to as Fe/N/C catalysts. During pyrolysis at
temperatures.gtoreq.800.degree. C., NH.sub.3 partly gasifies the
carbon support, resulting in a mass loss that depends on the
duration of the heat treatment (REFERENCE 18). The disordered
domains of the carbon support are preferentially gasified
(REFERENCE 19-21). As a result, micropores are created in the
carbon black particles. The mass loss (30-50 wt %) at which maximum
activity is reached corresponds to the largest microporous surface
area of the etched carbon, suggesting that these micropores
(size.ltoreq.2 nm) host the catalytic sites (REFERENCE 19). In
addition, the reaction of NH.sub.3 with the disordered carbon
domains also produces the N-bearing functionalities needed to bind
iron cations to the carbon support (REFERENCE 22-23).
[0009] Hence it has been proposed that each Fe/N/C catalytic site
comprises an iron cation coordinated by four pyridinic
functionalities attached to the edges of two graphene planes, each
belonging to adjacent crystallites on either side of a slit pore in
the carbon support (REFERENCE 19, 23). Thus, four factors have been
identified as requirements for producing active Fe-based catalysts
for ORR: (i) disordered carbon content in the catalyst precursor
(REFERENCE 18); (ii) iron; (id) surface nitrogen and (iv)
micropores in the catalyst. Disordered carbon allows for the
formation of micropores and nitrogen enrichment during pyrolysis in
NH.sub.3, Fe and N are essential because they form an integral part
of the catalytic site (REFERENCE 2), while micropores are required
to host the catalytic site (REFERENCE 19-21).
[0010] For NPMCs, it is meaningful to speak in terms of volumetric
activity for ORR. Conversion from Ag.sup.-1.sub.NPMC to
Acm.sup.-3.sub.electrode is described below. The volumetric
activity is the product of the catalytic site density and the
activity of a single site. The latter varies with voltage and is an
intrinsic property of the catalytic site. Therefore, if the site is
unchanged, increased volumetric activity can only be achieved by
increasing the site density.
[0011] The volumetric catalytic activity of a catalyst may be
marginally improved by increasing the Fe content. However, the
inventors previously found increased activity only up to a presence
of ca. 0.2 wt % Fe, beyond which the activity levels off and
eventually decreases (REFERENCE 24). Therefore, a nominal Fe
concentration of 0.2 wt % was at that time chosen for impregnation
onto pristine non-porous carbon blacks.
[0012] Furthermore, when catalysts were prepared using the
impregnation method on non-porous carbon black and pyrolysed in
pure NH.sub.3, the micropore surface area of the resulting
catalysts was shown to govern the catalytic activity because the
nitrogen and iron content were usually non-limiting (REFERENCE
19).
[0013] FeAc was therefore impregnated onto highly microporous
carbon supports followed by pyrolysis in NH.sub.3. Surprisingly,
this did not improve the activity as compared to catalysts made
with non-porous carbon supports. Instead, it was concluded that
only the micropores created during pyrolysis in NH.sub.3 may host
catalytic sites (REFERENCE 25). The inventors thus found that the
micropores in the as-received microporous carbon blacks do not bear
the surface nitrogen necessary to form catalytic sites.
Furthermore, since these carbon blacks have little disordered
carbon content, surface nitrogen is difficult to add during
pyrolysis in NH.sub.3.
[0014] REFERENCE 26 gives conditions for measuring the volumetric
activity of NPMCs in fuel cells.
[0015] REFERENCES 18 and 22 disclose the use of different carbon
blacks and activated carbon in catalysts of the prior art.
[0016] REFERENCES 27 and 28 disclose the use of a carbon pretreated
to add nitrogen and/or carbon with a nitrogen-containing molecule
with iron acetate in catalysts of the prior art.
[0017] REFERENCE 29 discloses the use of iron, cobalt, chromium and
manganese in catalysts of the prior art.
[0018] REFERENCES 30-38, 9, 39-46, discloses the use of at least
the following non-noble metal precursors in catalysts of the prior
art: cobalt porphyrin (Co tetramethoxyphenylporphyrin (TMPP)); iron
acetate, Fe tetramethoxyphenylporphyrin (TMPP) on pyrolysed
perylene-tetracarboxylic-dianhydride (PTCDA); Fe phthalocyanines;
Fe and Co tetraphenylporphyrin; Co phthalocyanines; Mo
tetraphenylporphyrin; metal/poly-o-phenylenediamine on carbon
black; metal porphyrin; molybdenum nitride; cobalt ethylene
diamine; hexacyanometallates; pyrrol, polyacrylonitrile and cobalt;
cobalt tetraazaannulene; and cobalt organic complexes.
[0019] REFERENCES 29, 56 and 57 report the successful use of
Fe.sup.II acetate, cobalt acetate, copper acetate, chromium
acetate, manganese acetate, nickel acetate, and ferrocene in prior
art catalysts.
[0020] The present inventors and other authors have successfully
used Fe.sup.II acetylacetonate, Fe.sup.II sulfate, Fe.sup.III
chloride, Fe.sup.III nitrate, Fe.sup.II oxalate, Fe.sup.III
citrate, ChloroFe tetramethoxyphenylporphyrine, cobalt
phthalocyanine, iron phthalocyanine, cobalt tetra-aza-annulene in
catalysts of the prior art.
[0021] The present inventors have successfully used all the
compounds between parenthesis and at least one compound of each
family bellow in non-noble catalysts of the prior art:
Phenanthroline (1,10-phenanthroline, Bathophenanthroline disulfonic
acid disodium salt hydrate, 4,7-Diphenyl and 5,6-dimethyl
phenanthroline, 4-aminophenanthroline); Phthalocyanine; Porphyrine;
Phthalonitrile (4-Amino-phthalonitrile); Melamine;
Hexaazatriphenylene; Tetracarbonitrile;
Benzene-1,2,4,5-tetracarbonitrile; amino-acids; Polypyrrole;
Polyaniline, Bismark Brown; and Bathocuproine (2,9
Dimethyl-4,7-diphenyl-1,10-phenantroline). These results were
unpublished before the present and were thus not part of the prior
art available to the skilled person.
[0022] There remains a need for improved NPMCs to replace the
Pt-based electrocatalysts used in PEMFCs.
[0023] The present description refers to a number of documents, the
content of which is herein incorporated by reference in their
entirety.
SUMMARY OF THE INVENTION
[0024] In accordance with the present invention, there is provided
a catalyst precursor comprising: (A) a microporous support; (B) a
non-noble metal precursor; and (C) a pore-filler, wherein the
micropores of the microporous support are filled with the
pore-filler and the non-noble metal precursor so that the micropore
surface area of the catalyst precursor is substantially smaller
than the micropore surface area of the support when the pore-filler
and the non-noble metal precursor are absent.
[0025] There is also provided a method for producing a catalyst
precursor, the method comprising: (A) providing a microporous
support; a non-noble metal precursor; and a pore-filler; and (B)
filling the micropores of the microporous support with the
pore-filler and the non-noble metal precursor so that the micropore
surface area of the catalyst precursor is substantially smaller
than the micropore surface area of the support when the pore-filler
and the non-noble metal precursor are absent.
[0026] In embodiments of the above catalyst precursor and method of
producing same, at least one of the microporous support, the
non-noble metal precursor, or the pore-filler is a nitrogen
precursor. As noted below, if none of the microporous support, the
non-noble metal precursor, or the pore-filler is a nitrogen
precursor, then the pyrolysis gas with which the catalyst precursor
will be treated should be a nitrogen precursor.
[0027] In embodiments of the above catalyst precursor and method of
producing same, the microporous support is highly microporous. In
embodiments, the microporous support is carbon-based. In
embodiments, the microporous support is carbon black or activated
carbon.
[0028] In embodiments of the above catalyst precursor and method of
producing same, the non-noble metal precursor is a precursor of
iron or cobalt. In specific embodiments, the non-noble metal
precursor is a precursor of iron. In embodiments, the catalyst
precursor has an iron loading of about 0.2 wt % or more based on
the total weight of the catalyst precursor. In more specific
embodiments, the catalyst precursor has an iron loading of about 1
wt % based on the total weight of the catalyst precursor.
[0029] In specific embodiments, the non-noble metal precursor is a
salt of a non-noble metal or an organometallic complex of a
non-noble metal. In more specific embodiments, the non-noble metal
precursor is Fe.sup.II acetate.
[0030] In embodiments of the above catalyst precursor and method of
producing same, the pore-filler comprises a polycyclic structure.
In specific embodiments, the pore-filler is
perylene-tetracarboxylic-dianhydride, 1,10-phenanthroline, perylene
tetracarboxylic-diimide, or polyacrylonitrile.
[0031] In embodiments of the above catalyst precursor and method of
producing same, the non-noble metal precursor and the pore-filler
are the same molecule.
[0032] In embodiments of the above catalyst precursor and method of
producing same, the micropore surface area of the catalyst
precursor is at most about 75%, 65%, 55%, 45%, 35%, 25% or 15% of
the micropore surface area of the support when the pore-filler and
the non-noble metal precursor are absent. In specific embodiments
of the above catalyst precursor and method, the micropore surface
area of the catalyst precursor is at most about 10% of the
micropore surface area of the support when the pore-filler and the
non-noble metal precursor are absent.
[0033] In embodiments of the above catalyst precursor and method of
producing same, the pore-filler/microporous support mass ratio is
50:50.
[0034] In embodiments of the above catalyst precursor and method of
producing same, the micropores of the microporous support are
filled with the pore-filler and the non-noble metal precursor by
ballmilling or resonant acoustic mixing with or without a grinding
medium. In specific embodiments, the ballmilling is planetary
ballmilling.
[0035] For certainty, in embodiments of the above method for
producing a catalyst precursor, the catalyst precursor is as
described above.
[0036] There is also provided a catalyst comprising the
above-described catalyst precursor, wherein the catalyst precursor
has been pyrolysed so that the micropore surface area of the
catalyst is substantially larger than the micropore surface area of
catalyst precursor, with the proviso that the pyrolysis is
performed in the presence of a gas that is a nitrogen precursor
when the microporous support, the non-noble metal precursor and the
pore-filler are not nitrogen precursors.
[0037] Finally, there is also provided a method of producing a
catalyst, the method comprising (A) providing a catalyst precursor
comprising a microporous support; a non-noble metal precursor; and
a pore-filler, wherein the micropores of the microporous support
are filled with the pore-filler and the non-noble metal precursor
so that the micropore surface area of the catalyst precursor is
substantially smaller than the micropore surface area of the
support when the pore-filler and the non-noble metal precursor are
absent; and (B) pyrolyzing the catalyst precursor so that the
micropore surface area of the catalyst is substantially larger than
the micropore surface area of catalyst precursor, with the proviso
that the pyrolysis is performed in the presence of a gas that is a
nitrogen precursor when the microporous support, the non-noble
metal precursor and the pore-filler are not nitrogen
precursors.
[0038] In embodiments of the above catalyst and method for
producing same, the micropore surface area of the catalyst is at
least half (50%), 60%, 70%, 75% or 80% of the micropore surface
area of the support when the pore-filler and the non-noble metal
precursor are absent. In specific embodiments, the micropore
surface area of the catalyst is at least three-quarters (75%) of
the micropore surface area of the support when the pore-filler and
the non-noble metal precursor are absent.
[0039] In embodiments of the above catalyst and method for
producing same, the mass loss during pyrolysis is about equal to
the pore-filler loading in the catalyst precursor (in weight %
based on the total weight on the catalyst precursor).
[0040] In embodiments of the above catalyst and method for
producing same, the catalyst has a nitrogen content of about 0.5 wt
% or more based on the total weight of the catalyst.
[0041] In embodiments of the above catalyst and method for
producing same, the catalyst is an oxygen reduction catalyst, a
catalyst for the disproportionation of hydrogen peroxide or a
catalyst for the reduction of CO.sub.2. In specific embodiments,
the catalyst is an oxygen reduction catalyst.
[0042] In embodiments of the above catalyst and method for
producing same, the pyrolysis is performed in a nitrogen-containing
reactive gas or vapor. In specific embodiments, the
nitrogen-containing reactive gas or vapor is NH.sub.3.
[0043] In embodiments of the above catalyst and method for
producing same, the pyrolysis is performed in an inert gas. In
embodiments, the inert gas is argon.
[0044] In specific embodiments of the above catalyst and method for
producing same, a second pyrolysis in a nitrogen-containing
reactive gas or vapor is performed following the pyrolysis
performed in the inert gas. In embodiments, the nitrogen-containing
reactive gas or vapor for the second pyrolysis is NH.sub.3.
[0045] In embodiments of the above catalyst and method for
producing same, the pyrolysis (either the first and/or the second
one if applicable) is performed at a temperature greater than about
600.degree. C.
[0046] For certainty, in embodiments of the above method for
producing a catalyst, the catalyst precursor and/or the catalyst
are as described above.
DETAILED DESCRIPTION OF THE INVENTION
[0047] It is an object of the present invention to provide
non-precious (non-noble) metal catalysts (NPMCs) for the oxygen
reduction reaction (ORR) in polymer electrolyte membrane fuel cells
(PEMFCs). Such catalyst may be referred to as a catalyst of the
type metal/N/C.
[0048] The present inventors were able to increase the number of
catalytic sites on the carbon of such NPMCs using a microporous
carbon (which had already lost most of its disordered carbon during
its manufacturing), as a starting material. More specifically, the
micropores of this microporous carbon were filled with a
pore-filler (and non-noble metal precursors). When reacting during
subsequent pyrolysis, the pore-filler and non-noble metal precursor
produce catalytic sites exactly where they are needed, i.e. in the
micropores of the carbon substrate.
[0049] The obtained catalysts thus contain a high density of active
sites. There may be different kinds of active sites in a same
catalyst but all active sites are believed to be as follows. First,
because of the use of pore-filler, each active site contains a
carbon poly-aromatic structure whose carbon atoms originate from
the pore-filler (see FIG. 1). The active sites contain at least one
non-noble metal atom and when there is more than one non-noble
metal atom, these non-noble metal atoms can be of same or different
nature. The active sites also contain at least about four nitrogen
atoms. Without being bound by theory, it is believed that the
nitrogen atoms are bound to the carbon atoms originating from the
pore-filler and/or to the metal atom(s), resulting in
pyridinic-type or pyrrolic-type N atoms. It is also believed that
the center of each active site is somewhat similar to the center of
porphyrin or phthalocyanine molecules, for which all nitrogen atoms
are of the pyrrolic-type. Finally, it is believed that the active
sites have an electronic contact with the walls of the micropores
(see FIG. 1).
[0050] The tests presented below have revealed that catalysts
obtained using this procedure have an activity in a fuel cell of 10
to 100 times higher than prior art catalysts obtained by simple
adsorption or impregnation on carbon. It is believed that the
components that have been successfully used in such prior art
catalysts that were obtained by simple adsorption or impregnation
(such as the microporous supports, the non-noble metal precursor,
the pyrolysis gas, and the like) will also be useful in the
catalysts of the invention.
[0051] Turning now to the present invention in more detail, there
is provided a catalyst precursor comprising: (A) a microporous
support; (B) a non-noble metal precursor; and (C) a pore-filler,
wherein the micropores of the microporous support are filled with
the pore-filler and the non-noble metal precursor so that the
micropore surface area of the catalyst precursor is substantially
smaller than the micropore surface area of the support when the
pore-filler and the non-noble metal precursor are absent.
[0052] As used herein a "catalyst" means a substance that initiates
or facilitates a chemical reaction; a substance that boosts the
kinetics of a given reaction. A "catalyst precursor" is a substance
from which a catalyst can be produced by pyrolysis. Herein,
"pyrolysis" means the transformation of a substance into one or
more other substances by heat in the presence or absence of a gas
(vacuum). As explained above, in the present invention, the
pore-filler in the catalyst precursor react during pyrolysis to
produce the desired catalytic sites in the catalyst.
[0053] As noted above, the catalyst precursors of the invention
comprise a microporous support. As used herein, a microporous
support is a support comprising micropores. For example, a
microporous support may have a micropore surface area of more than
about 100 m.sup.2/g. Herein, "micropores" refer to pores having a
size.ltoreq.2 nm. Most microporous supports usually also comprise
mesopores (between 2 and 50 nm in size) and macropores (having a
size>50 nm). As such, microporous supports have a "total"
surface area, which is provided by the micropores, the mesopores
and the macropores. As used herein the "micropore surface area" of
a substance is the surface area of this substance provided by its
micropores. The "total" surface area, micropore surface area,
mesopore surface area and macropores surface area can be determined
by methods well known in the art. For example, by measuring the
N.sub.2-adsorption isotherm and analyzing it with the Brunauer
Emett Teller (BET) equation and by applying non-local density
functional theory using a slit-pore model (Quantachrome software)
to determine pore size distribution.
[0054] In embodiments, the microporous support is a highly
microporous support. For example, a "highly microporous support"
may be a microporous support having a micropore surface area of
more than about 500 m.sup.2/g.
[0055] As will be appreciated by the person of skill in the art,
the exact nature of the microporous support is of little importance
to the invention as long as it comprises micropores. When the
catalyst is for use in fuel cells, the microporous support will be
conductive or able to be made conductive by methods known to
persons of skill in the art. A non-conductive microporous support
may be used, if it can be removed after fabrication of the
catalytic sites in its micropores, e.g. by acid leaching of the
non-conductive support, while leaving intact a skeleton of
carbon-based material with high site density. A non-limiting
example of such a process is the use of microporous silicon dioxide
and its acid leaching by hydrofluoric acid (HF). Hydrofluoric acid
is known to not attack such non-noble metal active sites (REFERENCE
49). Another non-limiting example of conductive microporous support
is a metal organic framework (MOF). The latter will decompose upon
pyrolysis and thus leave the desired skeleton of carbon-based
material.
[0056] In embodiments, the microporous or highly microporous
supports are carbon-based. Such carbon-based supports can be carbon
blacks, activated carbons, carbon nanotubes or nanostructures,
carbons derived from metallic carbides or oxides or from the
pyrolysis of polymers, or from any other method used to obtain
highly microporous carbons. In other embodiments, metal organic
frameworks may also be used as microporous supports. All the
microporous supports may be used as mixtures if desired.
[0057] In embodiments, the microporous support may be a nitrogen
precursor. As used herein, a "nitrogen precursor" is a molecule or
substance that provides one or more nitrogen atoms to the catalyst
after pyrolysis.
[0058] As noted above, the catalyst precursors of the invention
comprise a non-noble metal precursor. It is to be understood that a
mixture of non-noble metal precursors can be used. Any non-noble
metal precursor known to the skilled person to be useful in
catalysts of the prior art (i.e. those produced by adsorption or
impregnation) may be used.
[0059] As used herein, a "non-noble metal" is a metal other than a
noble metal. Noble metals are usually considered by the persons of
skill in the art to be ruthenium, rhodium, palladium, osmium,
iridium, platinum, and gold.
[0060] Examples of the non-noble metal include metals having atomic
numbers between 22 and 32, between 40 and 50 or between 72 and 82,
with the exclusion of atomic numbers 44-47 and 75-79. In
embodiments, the non-noble metal is iron, cobalt, copper, chromium,
manganese or nickel. In specific embodiments, the non-noble metal
is iron or cobalt. In more specific embodiments, the non-noble
metal is iron.
[0061] In embodiments, the catalyst precursor comprises between
about 0.05 and about 5.0 wt % of the non-noble metal based on the
total weight of the catalyst. In embodiments, the catalyst
precursor has a non-noble metal content, as provided by the
non-noble metal precursor, of about 0.2, 0.5, 1.0, 2.5, 3.0, 3.5,
4.0, or 4.5 wt % or more based on the total weight of the catalyst.
In embodiments, the catalyst precursor has a non-noble metal
content, as provided by the non-noble metal precursor, of about
5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, or 0.5 wt % or less
based on the total weight of the catalyst.
[0062] In embodiments, the catalyst precursor has an iron loading
of about 0.2 wt % or more based on the total weight of the catalyst
precursor. In more specific embodiments, the catalyst precursor has
an iron loading of about 1 wt % based on the total weight of the
catalyst precursor.
[0063] As used herein, a "non-noble metal precursor" is a molecule
that provides a non-noble metal atom to the catalyst during
pyrolysis. It is to be understood that a non-noble metal precursor
may contain only one non-noble metal or a mixture of several
non-noble metals. As noted above, the active sites of the catalyst
of the invention comprise at least one non-noble metal atom.
[0064] The non-noble metal precursor may be organometallic or
inorganic.
[0065] In embodiments, the non-noble metal precursor may be a salt
of the non-noble metal or an organometallic complex of the
non-noble metal. Generally, any metal precursor of size<2 nm, or
more generally of a size that allows it to be inserted or forced in
the micropores, would be suitable for use in the present invention.
Non-limiting examples of non-noble metal precursors include the
following broad classes with more specific examples in each class
given between parentheses: [0066] Metal acetates (Fe.sup.II
acetate, cobalt acetate, copper acetate, chromium acetate,
manganese acetate, nickel acetate); [0067] Metal acetylacetonate
(Fe.sup.II acetylacetonate); [0068] Metal sulfates (Fe.sup.II
sulfate); [0069] Metal chlorides (Fe.sup.III chloride); [0070]
Metal nitrates (Fe.sup.III nitrate); [0071] Metal oxalates
(Fe.sup.II oxalate); [0072] Metal citrates (Fe.sup.III citrate);
[0073] Fe(II) ethylene diammonium sulfate; [0074] Metal porphyrins
(Fe tetramethoxyphenylporphyrin, Fe 4-hydroxy-phenyl porphyrin,
mesotetra-phenyl Fe porphyrin, octaethyl Fe porphyrin, Fe
pentafluorophenyl porphyrin); [0075] Metallocene (Ferrocene,
cobaltocene); [0076] Metal--phthalocyanine (cobalt phthalocyanine,
iron phthalocyanine); [0077] Tetra-aza-annulene (cobalt
tetra-aza-annulene); [0078] Metal oxides; [0079] Metal nitrides;
[0080] Metal carbides; [0081] Metal sputtered over the microporous
support; and [0082] Mixtures of the above.
[0083] Other non-limiting examples of non-noble metal precursors
include: [0084] cobalt porphyrin: Co tetramethoxyphenylporphyrin
(TMPP); [0085] iron acetate, Fe tetramethoxyphenylporphyrin (TMPP)
on pyrolysed perylene-tetracarboxylic-dianhydride (PTCDA); [0086]
Fe phthalocyanines; [0087] Fe and Co tetraphenylporphyrin; [0088]
Co phthalocyanines; [0089] Mo tetraphenylporphyrin; [0090]
metal/poly-o-phenylenediamine on carbon black; [0091] metal
porphyrin; [0092] molybdenum nitride; [0093] cobalt ethylene
diamine; [0094] hexacyanometallates; [0095] pyrrol,
polyacrylonitrile and cobalt; [0096] cobalt tetraazaannulene; and
[0097] cobalt organic complexes.
[0098] In specific embodiments, the non-noble metal precursor is
Fe.sup.II acetate (FeAc).
[0099] In embodiments, the non-noble metal precursors may also be a
nitrogen precursor.
[0100] As noted above, the catalyst precursors of the invention
comprise a pore-filler. It is to be understood that a mixture of
pore-fillers can be used. It is also to be understood that the
non-noble metal precursor may be the pore-filler, i.e. the
non-noble metal precursor and the pore-filler may be the same
molecule.
[0101] As used herein, a "pore-filler" is a molecule that (1) is
small enough to enter (or be forced in) and fill the micropores of
the microporous support and (2) is carbon-based (i.e. organic) so
that it reacts during pyrolysis to produce in the micropores a
carbon poly-aromatic structure whose carbon atoms originate from
the pore-filler as illustrated in FIG. 1. As will be appreciated by
the person of skill in the art, the exact nature of the pore-filler
has therefore little importance to the present invention as long as
the pore-filler fulfills the above-noted requirements and
roles.
[0102] In embodiments, the pore-filler may comprise a polycyclic
structure, i.e. a structure made of rings (loops formed by a series
of connected carbon atoms), preferably aryl rings such as C.sub.6
rings, for example benzene. These rings may more easily construct
active sites and extend the graphite platelets that are found on
the edge of the graphitic crystallites within the microporous
support to provide the desired carbon poly-aromatic structure in
the micropores of the microporous support.
[0103] Different types of pore-fillers may be used. A first type
comprises molecules that contain carbon, but that do not contain
nitrogen atoms. Non-limiting examples of classes of such
pore-fillers include polycyclic aromatic hydrocarbons or their
derivatives. Non-limiting examples of pore-filler in these classes
include perylene or perylene tetracarboxylic dianhydride.
[0104] A second type of pore-filler comprises molecules that
contain both carbon and nitrogen atoms in their structure.
Non-limiting examples of classes of such pore-fillers include
phenanthrolines, melamine and cyanuric acid.
[0105] A last type of pore-filler comprises molecules that contain
carbon, nitrogen atoms and at least one metal atom in their
molecular structure. Non-limiting examples of classes of such
pore-fillers include metal-phenanthroline complexes,
metal-phthalocyanines, and metalporphyrins.
[0106] It is to be understood that the pore-filler may be any
combination of pore-fillers from the first, second and/or third
above-described types of pore-fillers.
[0107] In embodiments, the pore-filler may be a nitrogen
precursor.
[0108] Non-limiting examples of pore-fillers that also are nitrogen
precursors include the following broad classes with specific
examples given between parenthesis: [0109] Phenanthroline
(1,10-phenanthroline, Bathophenanthroline disulfonic acid disodium
salt hydrate, 4,7-Diphenyl and 5,6-dimethyl phenanthroline,
4-aminophenanthroline); [0110] Phthalocyanine; [0111] Porphyrine;
[0112] Pyrazine (Tetra 2 pyridinyl pyrazine,
dihydropyridylpyridazine); [0113] Phthalonitrile
(4-Amino-phthalonitrile); [0114] Pyridine (2,2':6',2''-Terpyridine,
4'-(4-Methylphenyl)-2,2':6',2''-terpyridine,
6,6''-Dibromo-2,2':6',2''-terpyridine,
6''-Dibromo-2,2':6',2''-terpyridine, aminopyridines) [0115]
Melamine; [0116] Tetra-aza-annulene; [0117] Hexaazatriphenylene;
[0118] Tetracarbonitrile; [0119] Benzene-1,2,4,5-tetracarbonitrile
[0120] 6-Pyridin-2-yl-[1,3,5]triazine-2,4-diamine; [0121] All
amino-acids; [0122] Polypyrrole; and [0123] Polyaniline.
[0124] Non-limiting examples of pore-fillers that do not contain
nitrogen atoms and are thus not nitrogen precursors include the
following broad classes with specific examples given between
parenthesis [0125] Perylene [perylene-tetracarboxylic-dianhydride
(PTCDA)]; [0126] Cyclohexane; [0127] Benzene; [0128] Toluene;
[0129] Pentacene; [0130] Coronene; [0131] Graphite transformed into
disordered carbon of size<2 nm by ballmilling; [0132] Polycyclic
aromatics (including perylene, pentacene, coronene, etc.); and
[0133] Coal tar or petroleum pitch (these are raw materials for a
commercial process for carbon fiber production and are high in
polycyclic aromatics).
[0134] In embodiments, the pore-filler is
perylene-tetracarboxylic-dianhydride, 1,10-phenanthroline, perylene
tetracarboxylic-diimide, or polyacrylonitrile.
[0135] As described above, the pore-filler enters and fills the
micropores of the microporous support. In the catalyst precursor of
the invention, the micropores of the microporous support are filled
with the pore-filler and the non-noble metal precursor. As used
herein, "the micropores are filled" does not mean that all the
micropores are completely full, it rather means that a substantial
portion of the micropores are at least partially filled. The
micropores may be considered as being filled if for example 75% or
more of them are completely full and 25% or less of them are empty
or if 75% or more of them is mostly (at least half) full.
[0136] One observable effect of the micropores being filled is that
the micropore surface area of the catalyst precursor becomes
substantially smaller than the micropore surface area of the
support when the pore-filler and the non-noble metal precursor are
absent. Indeed, as can be seen from FIG. 1 and FIG. 4, filled (or
even partially filled) micropores have a smaller surface area than
empty micropores. Therefore, the micropore surface area of the
catalyst precursor where the micropores are filled will be
substantially smaller than the micropore surface area of the
microporous support with empty micropores.
[0137] In embodiments, the micropore surface area of the catalyst
precursor is at most about 75% of the micropore surface area of the
support when the pore-filler and the non-noble metal precursor are
absent. In more specific embodiments, the micropore surface area of
the catalyst precursor is at most about 60%, 50%, 40%, 30%, 20%, or
10% of the micropore surface area of the support when the
pore-filler and the non-noble metal precursor are absent.
[0138] In embodiments, the pore-filler/microporous support mass
ratio is 50:50 (as calculated using the weight of the pore-filler
and the weight of the microporous support in the catalyst
precursor).
[0139] The present invention also relates to a catalyst comprising
the above catalyst precursor, wherein the catalyst precursor has
been pyrolysed so that the micropore surface area of the catalyst
is substantially larger than the micropore surface area of catalyst
precursor, with the proviso that the pyrolysis is performed in the
presence of a gas that is a nitrogen precursor when the microporous
support, the non-noble metal precursor and the pore-filler are not
nitrogen precursors.
[0140] As explained above, it is believed that the active catalytic
sites of the present catalyst comprise a carbon poly-aromatic
structure whose carbon atoms originate from the pore-filler, at
least one non-noble metal atom and at least four nitrogen atoms.
There must therefore be nitrogen atoms provided to the catalyst.
These nitrogen atoms can be provided by the microporous support,
the non-noble metal precursor, the pore-filler and/or the pyrolysis
gas. In this case, these components are also nitrogen precursor as
explained above.
[0141] When the microporous support, the non-noble metal precursor
and the pore-filler are not nitrogen precursors, the necessary
nitrogen atoms are provided by a gas used during pyrolysis.
Therefore in that case, the gas itself is a nitrogen precursor.
[0142] As explained above, in the present invention, the
pore-filler of the catalyst precursor is believed to react during
pyrolysis to produce the desired catalytic sites in the catalyst.
More specifically, pyrolysis of the catalyst precursor causes the
pore-filler to react and produce a carbon poly-aromatic structure
whose carbon atoms originate from the pore-filler in the
micropores. This results in the construction of active catalytic
sites and the extension of the graphite platelets that are found on
the edge of the graphitic microporous support. The pyrolysis also
causes the non-noble metal precursor and the nitrogen precursor (be
it the microporous support, the non-noble metal precursor, the
pore-filler or the gas used for pyrolysis) to react and give up
their non-noble metal and nitrogen atoms to the catalytic site. The
active catalytic sites are thus formed from the carbon from the
pore-filler, the nitrogen from the nitrogen precursor and the
non-noble metal from the non-noble metal precursor. This whole
process is illustrated in FIG. 1. More detail on the pyrolysis
procedure will be given below.
[0143] As can be seen from FIG. 1, as the pore-filler, the nitrogen
precursor and the non-noble metal react (i.e. decompose and
partially go away) during pyrolysis, the micropores of the
microporous support are more or less restored. Therefore, the
micropore surface area of the catalyst becomes substantially larger
during pyrolysis. In other words, the micropore surface area of the
catalyst is substantially larger than the micropore surface area of
catalyst precursor. In the extreme, the micropore surface area of
the catalyst may be almost as high as the micropore surface area of
the microporous support when pore-filler and the non-noble metal
precursor were absent.
[0144] In embodiments, the micropore surface area of the catalyst
is at least about 50% of the micropore surface area of the support
when the pore-filler and the non-noble metal precursor are absent.
In embodiments, the micropore surface area of the catalyst is at
least about 60%, 70%, 75% or 80% of the micropore surface area of
the support when the pore-filler and the non-noble metal precursor
are absent.
[0145] As such, it should be understood that the pyrolysis cause a
loss of mass. The catalyst obtained after pyrolysis is lighter than
the catalyst precursor. In embodiments, the mass loss during
pyrolysis is about equal to a pore-filler loading in the catalyst
precursor (in weight % based on the total weight on the catalyst
precursor).
[0146] The non-noble metal content of the catalyst after pyrolysis
may be measured by methods known in the art, for example neutron
activation analysis.
[0147] The catalyst may comprise between about 0.5 to about 10.0 wt
% of the nitrogen based on the total weight of the catalyst. In
embodiments, the catalyst has a nitrogen content, as provided by
the nitrogen precursor, of about 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0,
7.0, 8.0, or 9.0 wt % or more based on the total weight of the
catalyst. In embodiments, the catalyst has a nitrogen content, as
provided by the nitrogen precursor, of about 9.0, 8.0, 7.0, 6.0,
5.0, 4.0, 3.0, 2.0, 1.0 wt % or less based on the total weight of
the catalyst. This nitrogen content may be measured by methods
known in the art, for example, x-ray photoelectron
spectroscopy.
[0148] If the microporous support is carbon-based, the carbon
content is usually about 80 wt % or more based on the total weight
of the catalyst. The catalyst may comprise between about 80 and
about 99.9 wt % of carbon. It is to be noted that carbon usually
comprises some oxygen (usually between 0.5 and 5% wt). If the
microporous support does not contain carbon, the carbon content of
the catalyst may be low since the carbon content will be provided
only by from the pore-filler (and optionally the non-noble metal
precursor) used to fill the microporous support.
[0149] In embodiments, the catalyst is an oxygen reduction
catalyst, a catalyst for the disproportionation of hydrogen
peroxide or a catalyst for the reduction of CO.sub.2. Indeed, it is
believed that the present catalysts will be useful for the
disproportionation of hydrogen peroxide and the reduction of
CO.sub.2 because it is known for non noble metal catalysts obtained
from heat treatment or without heat treatment (metal-N.sub.4
molecules as phthalocyanines) that the activity for the O.sub.2
electro-reduction reaction and for the chemical disproportionation
of H.sub.2O.sub.2 follow the same trend, i.e. if a catalyst shows
high activity for one reaction, it will show high activity for the
other reaction as well (REFERENCE 50). This fact has also been
verified by the present inventors and improved H.sub.2O.sub.2
disproportionation reaction has been measured on a catalyst of the
invention. Further, it is also known that electroreduction of
CO.sub.2 is catalyzed by metal macrocycles in which a metal ion is
coordinated to 4 nitrogen atoms located in a polyaromatic frame, a
structure similar to that proposed for the present catalytic sites
used for the reduction of oxygen (REFERENCE 51).
[0150] In more specific embodiments, the catalyst is an oxygen
reduction catalyst. Such a catalyst will be useful at the cathode
of various low temperature fuel cells, including principally
polymer electrolyte membrane (PEM) such as H.sub.2/O.sub.2 polymer
electrolyte membrane fuel cells, direct alcohol fuel cells, direct
formic acid fuel cells and even alkaline fuel cells. Such a
catalyst may also be useful at the cathode of various primary and
secondary metal-air batteries, including zinc-air batteries.
[0151] The present invention also relate to methods of producing
the above-described catalyst precursors and catalysts.
[0152] Therefore, the present invention relates to a method for
producing a catalyst precursor, the method comprising (A) providing
a microporous support; a non-noble metal precursor; and a
pore-filler; and (B) filling the micropores of the microporous
support with the pore-filler and the non-noble metal precursor so
that the micropore surface area of the catalyst precursor is
substantially smaller than the micropore surface area of the
support when the pore-filler and the non-noble metal precursor are
absent.
[0153] It should be noted that prior art methods of adding nitrogen
and/or non-noble metal precursors to the support did not result in
the filling of the micropores as in the present invention. Indeed,
the prior art methods for adding these nitrogen and/or non-noble
metal precursors to catalyst precursors typically involved
impregnation or adsorption. There are inherent solubility and
adsorbability limitations to these methods that prevent the filling
of the micropores so that that the micropore surface area of the
catalyst precursor is substantially smaller than the micropore
surface area of the support when the pore-filler and the non-noble
metal precursor are absent. Furthermore, in the prior art, the
nitrogen and/or non-noble metal precursors were sometimes added to
non-microporous supports to produce catalyst precursors.
[0154] In the present invention, the addition of the pore-filler
and the non-noble metal precursors to the microporous support is
carried out so that the micropores of the support are filled and
the micropore surface area of the catalyst precursor is
substantially smaller than the micropore surface area of the
support when the pore-filler and the non-noble metal precursor are
absent. The limitation associated with the prior art
impregnation/adsorption methods are thus overcome as the micropores
filled using different methods. Non-limiting examples of such
methods include any form of ballmilling or reactive ballmilling,
including but not limited to planetary ballmilling, and resonant
acoustic mixing.
[0155] Planetary ballmilling is a low-energy material processing
technique involving a container with grinding media that rotates in
a planet-like motion. It uses both friction and impact effects to
force the pore-filler and the non-noble metal precursor into the
micropores of the microporous support, while leaving its
microstructure relatively unaffected. The ballmilling may be
performed on dry powders of non-noble metal precursor, the
pore-filler and the microporous support. Alternatively, ballmilling
may be performed in wet conditions with the non-noble metal
precursor and the pore-filler in solution and the microporous
support in suspension in this solution.
[0156] Resonant mixing is a method that uses low-frequency
high-intensity sound energy for mixing. It may be carried out with
or without grinding media.
[0157] Therefore, in embodiments of the present invention, the
micropores of the microporous support are filled with the
pore-filler and the non-noble metal precursor by ballmilling or by
acoustic mixing with or without grinding media. In more specific
embodiments, the ballmilling is planetary ballmilling.
[0158] It is to be understood that the non-noble metal precursor
and the pore-filler may be introduced in the micropores of the
microporous support either together or separately.
[0159] The present invention also relates to a method of producing
a catalyst, the method comprising (A) providing a catalyst
precursor comprising a microporous support; a non-noble metal
precursor; and a pore-filler, wherein the micropores of the
microporous support are filled with the pore-filler and the
non-noble metal precursor so that the micropore surface area of the
catalyst precursor is substantially smaller than the micropore
surface area of the support when the pore-filler and the non-noble
metal precursor are absent; and (B) pyrolyzing said catalyst
precursor so that the micropore surface area of the catalyst is
substantially larger than the micropore surface area of catalyst
precursor, with the proviso that the pyrolysis is performed in the
presence of a gas that is a nitrogen precursor when the microporous
support, the non-noble metal precursor and the pore-filler are not
nitrogen precursors.
[0160] The atmosphere in which the pyrolysis is performed may be:
[0161] a nitrogen-containing reactive gas or vapor, non-limiting
examples of which being NH.sub.3, HCN, and CH.sub.3CN; [0162] an
inert gas, non-limiting examples of which being N.sub.2, Ar, and
any other inert gas or vapor; or [0163] a mixture of a
nitrogen-containing reactive gas or vapor and an inert gas.
[0164] As used herein a nitrogen-containing reactive gas or vapor
is a nitrogen-containing gas or vapor that will react during
pyrolysis to provide a nitrogen atom to the catalyst. A
non-limiting example of a nitrogen-containing gas or vapor that
does not so react is N.sub.2.
[0165] As used herein, an inert gas is a gas that will not react
with the catalyst precursor/catalyst at the pyrolysis temperature,
an example of which is argon.
[0166] Therefore, in embodiments the pyrolysis is performed in a
nitrogen-containing reactive gas or vapor. In specific embodiments,
the nitrogen-containing reactive gas or vapor is NH.sub.3.
[0167] In other embodiments, the pyrolysis is performed in an inert
gas. In embodiments, the inert gas is argon.
[0168] In embodiments where the pyrolysis is performed in an inert
gas, a second pyrolysis in a nitrogen-containing reactive gas or
vapor is performed following the pyrolysis performed in the inert
gas.
[0169] As explained above however, there must be nitrogen atoms
provided to the catalyst. These nitrogen atoms can be provided by
the microporous support, the non-noble metal precursor, the
pore-filler and/or the gas used for pyrolysis. When the microporous
support, the non-noble metal precursor and the pore-filler are not
nitrogen precursors, the necessary nitrogen atoms must be provided
by the gas used during pyrolysis (either the first one, or the
second if present). Therefore in that case, the pyrolysis gas
itself is a nitrogen precursor. In specific embodiments, the gas
that is a nitrogen precursor is a nitrogen-containing reactive gas
or vapor, non-limiting examples thereof being NH.sub.3, HCN, and
CH.sub.3CN.
[0170] The time and temperature required for the pyrolysis will be
easily determined by the person of skill in the art. More
specifically, the pyrolysis may be performed at temperatures
ranging from about 300 to about 1200.degree. C. In specific
embodiments, the pyrolysis is performed at a temperature greater
than about 600.degree. C.
[0171] Optionally, a final pyrolysis under H.sub.2 may be carried
out to eliminate excess heteroatoms like (N, O, S, etc . . . ) in
the catalyst if desired.
[0172] Optionally, acid etching may be used to remove excess
non-noble metal before or after any pyrolysis.
[0173] Post-pyrolysis ballmilling can optionally be carried out to
control particle size formed during the pyrolysis.
[0174] For use in a fuel cell, the catalyst is processed in order
to form part of the cathode of the fuel cell. This is typically
accomplished by thoroughly mixing the catalyst and an ionomer like
Nafion. The Nafion to catalyst mass ratio has to be adjusted and
depends on the catalyst, but can be easily determined by the person
of skill in the art. The optimal ratio may range between about 1
and about 4. Given the fact that the present catalysts are much
less expensive than noble metal or platinum-based catalysts, the
current density of the fuel cell may be increased by increasing the
loading of the former with little effect on cost. Therefore, the
loading of present catalysts may be increased until mass transport
losses become unacceptable.
[0175] If the electron conductive properties of the obtained
catalysts are not sufficient for optimal performance in fuel cell,
a given ratio of a conductive powder [carbon black or any electron
conductive powder that does not corrode in acid medium (for all PEM
fuel cells) or alkaline medium (for alkaline fuel cell)] may be
added.
[0176] Herein, "about" means plus or minus 5% of any numerical
value it qualifies.
[0177] Other objects, advantages and features of the present
invention will become more apparent upon reading of the following
non-restrictive description of specific embodiments thereof, given
by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0178] In the appended drawings:
[0179] FIG. 1 is a schematic representation of catalytic site
formation in the micropores of the carbon support: (a) Simplified
3-D view of a slit pore between two adjacent graphitic crystallites
in the carbon support; (b) Plan view of an empty slit pore between
two crystallites; (c) Plan view of a slit pore filled with
pore-filler and metal precursor after planetary ballmilling; and
(d) Plan view of the presumed catalytic site (incomplete) and
graphene layer growth (shaded aromatic cycles) between two
crystallites after pyrolysis.
[0180] FIG. 2 shows the volumetric current density of best
non-precious metal catalyst described below. Polarization curves
(converted to P.sub.O2 1 bar, 100% RH, 80.degree. C.) from
H.sub.2--O.sub.2 fuel cell testing for cathodes made with the best
non-precious metal catalyst (NPMC) below (hollow circles) and for
reference purposes, the presumed previous best NPMC (REFERENCE 7)
(hollow diamonds). A catalyst loading of ca. 1 mgcm.sup.-2 was used
for both NPMC polarization curves. The actual Fe content in the
catalyst from below is 1.7 wt %, resulting in a Fe loading of 17
.mu.gcm.sup.-2. The volumetric (kinetic: free of diffusion or
charge transport effects that are not related to the catalytic
activity of the catalyst) current density at 0.8 V iR-free cell
voltage is the intersection of the extended Tafel slope of the
polarization curves (dashed lines) with the 0.8 V axis. Also
included are the 2010 (filled star) and 2015 (filled hexagon) U.S.
DOE performance targets for ORR on NPMCs, all at the reference
conditions of PO2 1 bar, 100% RH and 80.degree. C.
[0181] FIG. 3 is a comparison of best non-precious metal catalyst
described below with a Pt-based catalyst. Polarization curves from
H.sub.2--O.sub.2 fuel cell testing (P.sub.O2 and P.sub.H2 1.5 bar,
100% RH, 80.degree. C.) for cathodes made with the best
non-precious metal catalyst below (two different catalyst loadings)
and a ready-to-use Gore PRIMEA.TM. 5510 MEA from W. L. Gore &
Associates with ca. 0.4 mg Pt cm.sup.-2 at cathode and anode (top
line as indicated). Flow rates for H.sub.2 and O.sub.2 were well
above stoichiometric. The actual Fe content in our catalyst is 1.7
wt %, resulting in a Fe loading of 17 .mu.gcm.sup.-2 for a catalyst
loading 1 mg cm.sup.-2.
[0182] FIG. 4 shows the micropore surface area (hollow squares) and
kinetic current at 0.8 V.sub.iR-free cell voltage obtained from
fuel cell testing (hollow circles) vs mass loss (during pyrolysis)
for catalysts produced with a 50% nominal concentration of PCTDA as
the pore-filler and a nominal Fe content of 0.2 wt %. The kinetic
current, in Ag.sup.-1 at 0.8V.sub.iR-free under 1.5 bar O.sub.2
corresponding to the optimal mass loss upon pyrolysis, is reported
in Table 2. Black Pearls 2000.TM. was used as the carbon support.
Pyrolysis was conducted in NH.sub.3 at 1050.degree. C. Catalyst
loading for all tests was ca. 1 mgcm.sup.-2. The Nafion-to-Catalyst
ratio was 2. Micropore surface areas of pristine and pore-filled
carbon support are represented by the filled triangle and filled
star, respectively.
[0183] FIG. 5 shows polarization curves from fuel cell testing
(P.sub.O2 1.5 bar, 100% RH, 80.degree. C.) for catalysts produced
with a nominal Fe content of 0.2 wt % and various nominal
concentrations of PTCDA as the pore-filler: 0% (fourth line from
the top when looking at the left side of the graphic, 25% (third
line), 50% (second line) and 75% (first line). Black Pearls
2000.TM. was used as the carbon support. Pyrolysis was conducted in
NH.sub.3 at 1050.degree. C. Catalyst loading for all tests was ca.
1 mgcm.sup.-2. The Nafion-to-Catalyst ratio was 2.
[0184] FIG. 6 shows the kinetic activity at 0.8V iR-free, nitrogen
content, and micropore surface area for catalysts made with 50% BP,
50% PTCDA and 1 wt % of iron, pyrolysed using method I
(1050.degree. C. in NH.sub.3).
[0185] FIG. 7 shows the nitrogen content, micropore surface area
and ORR kinetic activity in fuel cell for catalysts made using
method I (NH.sub.3 pyrolysis at 1050.degree. C.) having (A) various
mass ratios of PTCDA in the catalyst precursor and a fixed nominal
iron loading of 0.2 wt %; and (B) fixed mass ratio of 50 wt % PTCDA
in the catalyst precursor with various nominal iron loadings. Upper
graphs: nitrogen content (squares, read on left hand-side Y-scale)
and micropore surface area (stars, read on right hand-side
Y-scale). Lower graphs: Kinetic current per mass of catalyst
measured at 0.9V iR-free voltage (upright triangles) and 0.8V
iR-free voltage (inverted triangles).
[0186] FIG. 8 shows the nitrogen content, micropore surface area
and ORR kinetic activity in fuel cell for catalysts made using
method II (Ar pyrolysis at 1050.degree. C. for 60 min and then
NH.sub.3 pyrolysis at 950.degree. C.) having (A) various mass
ratios of phen in the catalyst precursor and a fixed nominal iron
loading of 1 wt %; and (B) fixed mass ratio of 50 wt % phen in the
catalyst precursor with various nominal iron loadings. Upper
graphs: nitrogen content (squares, read on left hand-side Y-scale)
and micropore surface area (stars, read on right hand-side
Y-scale). Lower graphs: Kinetic current per mass of catalyst
measured at 0.9V iR-free voltage (upright triangles) and 0.8V
iR-free voltage (inverted triangles).
[0187] FIG. 9 shows the PEM fuel cell polarization curves for
Fe-based catalysts made with various combinations of carbon
black/pore-filler. In all cases the pore-filler mass ratio and
nominal iron content was 50% and 1 wt %, respectively. Inset:
iR-free voltage vs. current per cm.sup.-2, semi-logarithmic plot.
All catalyst precursors were subjected to one pyrolysis in NH.sub.3
at 1050.degree. C. except the catalyst made with BP+phen. The
latter was subject to two pyrolyses, the 1.sup.st in Ar and the
2.sup.nd in ammonia (see experimental). H.sub.2/O.sub.2, 1 bar back
pressure, 100% relative humidity, cell at 80.degree. C., 1.14
cm.sup.2 geometric area of electrode.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0188] The present invention is illustrated in further details by
the following non-limiting examples.
Example 1
[0189] The embodiments of the invention described below elevates
the catalytic activity of iron-based NPMCs by a factor>25
compared to the previously best reported activity (REFERENCE 7);
high enough to equal Pt-based cathodes with loadings.ltoreq.0.4
mgPtcm.sup.-2, at cell voltage.gtoreq.0.9 V. The results presented
below show that sufficiently active and inexpensive NPMCs for the
ORR are possible. The present NPMCs will be useful for ORR in
direct alcohol, formic acid and alkaline fuel cells.
[0190] To capitalize on the high micropore content of microporous
carbon blacks and overcome the limitation due to their lack of
disordered carbon, these micropores were filled with a mixture of
pore-filler (PF) and iron precursor. Doing so creates a catalyst
precursor that complies with the four factors required for
producing active NPMCs, as described above. This innovative concept
is illustrated in FIG. 1. To overcome the limitation of solubility
and/or adsorbability associated with the impregnation method,
planetary ballmilling was used to fill the pores of the microporous
carbon support with various PFs and metal precursor.
[0191] In the present, the chosen microporous carbon black
(micropore surface area 934 m.sup.2g.sup.-1) and iron precursor
used for all catalysts is Black Pearls 2000.TM. by Cabot (BP) and
iron.sup.II acetate (FeAc), respectively. Two pore-fillers were
used. The first, perylene-tetracarboxylic-dianhydride (PTCDA) is
nitrogen free. The second, 1,10-phenanthroline (Phen) is N-bearing.
For catalysts made using PTCDA, the N atoms necessary to form
catalytic sites arise from its reaction with NH.sub.3 during
pyrolysis. For catalysts made using Phen, the pyrolysis was
performed in either Ar or NH.sub.3 as Phen already contains
nitrogen. It is also worthy to note that Phen forms a complex with
Fe.sup.2+.
[0192] Catalyst Synthesis. Catalyst precursors refer to the powder
mixtures prepared to be subsequently pyrolysed. All prepared
catalyst precursors consist of a carbon support, a pore-filler and
a non-noble metal precursor. These powder mixtures were prepared
using the planetary ballmilling (PBM) method. The carbon support
and the non-noble metal precursor used for all catalysts are BP and
FeAc, respectively. The PFs used are PCTDA or Phen, which are
represented as molecules (a) and (b) below respectively.
##STR00001##
[0193] For catalyst precursors containing PTCDA, the three
materials (BP, PTCDA and FeAc) were placed in a hardened steel vial
(ca. 65 cm.sup.3) with 20 chrome steel balls of 0.25 inch diameter.
Typically, ca. 1 g of powder is ballmilled at once. The
ball-to-powder ratio was ca. 20:1. The vial was purged of air and
filled with nitrogen using a glovebox. Once tightly sealed, the
vial was placed in a planetary ballmiller (FRITSCH Pulverisette 7)
to undergo 3 hours of ballmilling at 400 rpm. The resulting dry
powder formed the catalyst precursor.
[0194] For catalyst precursors containing Phen, the Phen and FeAc
were first mixed in a solution of ethanol to form a
[Fe(Phen).sub.3].sup.2+ complex. This was evidenced by the deep red
color that emerged from an otherwise clear solution. The carbon
support, BP, was then added to the solution. This solution was
stirred over a magnetic hotplate for ca. 2 hours and then placed in
a drying oven overnight to be completely dried in air at ca.
90.degree. C. Once dried the powder was placed in a steel vial and
underwent the same ballmilling processing steps as described
earlier for the catalyst precursor containing PCTDA. The resulting
dry powder formed the catalyst precursor.
[0195] To become active ORR catalysts all catalyst precursors
underwent one or two pyrolyses in either Ar or NH.sub.3 depending
on the PF used. The pyrolysis procedures to obtain the catalytic
activities reported in Tables 2 and 3 are described below.
[0196] Methods. Surface area measurement of catalysts was performed
with a Quantachrome Instruments Autosorb-1 and with N.sub.2 as an
adsorbate. Isotherm analysis was performed as in REFERENCE 19.
Surface elemental analysis of catalysts was performed by X-ray
photoelectron spectroscopy using a VG Escalab 200i instrument. The
Al K.sub..alpha. line (1486.6 eV) was chosen as the X-ray source.
The quantification of the elements was performed using Casa
software. Bulk Fe content of the best catalyst was measured with
neutron activation analysis at Ecole Polytechnique de Montreal.
MEAs were prepared using a Nafion 117 membrane and hot-pressing of
the anode, cathode and membrane was done at ca. 140.degree. C. for
40 seconds.
[0197] Electrochemical characterization. Two catalyst ink
formulations were used. One formulation resulted in a
Nafion-to-catalyst ratio (NCR) of 2 and another 1.5. For catalyst
inks with a NCR of 2, 10 mg of catalyst was mixed in a glass vial
with 435 .mu.L of Nafion 5 wt % (Aldrich), 54 .mu.L of ethanol and
136 .mu.L of nanopure water. For catalyst inks with a NCR of 1.5,
10 mg of catalyst was mixed in a glass vial with 326 .mu.L of
Nafion 5 wt % (Aldrich), 163 .mu.L of ethanol and 136 .mu.L of
nanopure water. In both cases the inks were first sonicated for 30
minutes, agitated in a vortex mixer for 15 minutes, sonicated once
more for 15 minutes and finally agitated for 5 more minutes. These
catalysts inks were then used to prepare the cathode for fuel cell
testing.
[0198] Preparation of Cathodes and Anodes. The cathodes used for
fuel cell testing were prepared using the catalyst inks described
above. To obtain a loading of ca. 1 mgcm-2 of catalyst, 71 .mu.L of
ink was deposited on a round uncatalysed 1.14 cm.sup.2 substrate,
or gas diffusion media (BASF ELAT). For some fuel cell tests,
however, higher loadings were deposited. The anode used for all
fuel cell tests performed with NPMCs at the cathode was the
catalyst layer of a BASF ELAT substrate coated with 0.5 mg
Ptcm.sup.-2, 20 wt % Pt/Vu. The active side (catalyst layer) of the
substrate was brushed with a thin layer of Nafion 5 wt % solution
(ca. 0.5 mgcm.sup.-2). The anode and cathode were then placed in a
vacuum oven at ca. 80.degree. C. to dry for 1 hour. The mass of
catalyst in the cathode was determined by subtracting the mass of
the uncatalysed substrate from the dry mass of the catalyst-coated
substrate and dividing the remainder by (NCR+1).
[0199] Fuel Cell Testing. Membrane electrode assemblies (MEAs) were
tested in a single-cell test fuel cell (Electrochem Inc.) and the
experiments were controlled with a potentiostat PARSTAT.TM. 2273
(Princeton Applied Research).
[0200] Teflon gaskets were used at both the anode and cathode
sides. The gasket thicknesses were chosen to obtain ca. 25%
compression of the gas diffusion+catalyst layers.
[0201] First, the fuel cell and H.sub.2/O.sub.2 humidifier
temperatures were raised and maintained to 80.degree. C. and
105.degree. C./95.degree. C., respectively, under N.sub.2 flows.
Once set temperatures were reached, pure H.sub.2 and O.sub.2 were
then fed for ca. 15 minutes. Back pressures were set to ca. 1 bar
for both anode and cathode sides. Thus, the absolute pressure for
both H.sub.2 and O.sub.2 is 1.5 bars (with ca. 0.5 bar partial
pressure attributed to water vapor at ca. 80.degree. C.). Flow
rates for H.sub.2 and O.sub.2 were well above stoichiometric.
[0202] First an electrochemical impedance spectroscopy (EIS)
measurement was made at open circuit voltage (OCV), with
frequencies ranging from 50 kHz down to 10 Hz. Typical resistance
values of ca. 0.2 .OMEGA.cm.sup.2 were obtained. A polarization
curve was then recorded by scanning the cell voltage from OCV down
to 0 V at a scan rate of 0.5 mVs.sup.-1.
[0203] Results and Discussion. First, the effect of the wt % of
PTCDA in the catalyst precursor was investigated. Four different wt
% PTCDA (0, 25, 50, 75) were used with a constant nominal Fe
loading of 0.2 wt %. Optimal volumetric activities of 1.8, 8.5, 22,
and 27 Acm.sup.-3 were obtained, respectively. The experimental
conditions and corresponding fuel cell polarization curves are
given in FIG. 5. Following these experiments, a pore filler loading
of 50 wt % (PTCDA or Phen) was chosen to investigate the effect of
nominal Fe loading in the catalyst precursor. Catalyst precursors
made with PTCDA were pyrolysed in NH.sub.3 and those made with Phen
in Ar, both at 1050.degree. C. While better volumetric activities
were obtained with the PTCDA series (Table 1), the effect of a
subsequent 5 minute pyrolysis in NH.sub.3 for the Phen series was
investigated. This subsequent pyrolysis amplified the volumetric
activity of the Ar-pyrolysed Phen series up to 20 times. These
amplified activities surpass those of the PTCDA series. Two factors
were further optimized on the most active catalyst corresponding to
1 wt % nominal Fe content (84 Acm.sup.-3, Table 1): (i) the mass
loss during pyrolysis in NH.sub.3 and (ii) the effect of the
Nafion-to-catalyst ratio (NCR) in the cathode. The optimal mass
loss and NCR were found to be ca. 30% and 1.5, respectively,
leading to an increase in volumetric activity from 64 to 99
Acm.sup.-3, much closer to the 2010 U.S. DOE performance target for
ORR on NPMCs. Additional details on mass activity, mass loss during
pyrolysis, micropore surface area and nitrogen content may be found
in Table 2. Details of methods used for catalyst synthesis, MEA
preparation and fuel cell testing may be found below.
TABLE-US-00001 TABLE 1 Catalytic activities for optimized
catalysts. Activities at 0.8 V vs RHE obtained from fuel cell
testing at 80.degree. C. and under 1.5 bar O.sub.2. Black Pearls
2000 .TM. was used as the carbon support. The mass ratio of
pore-filler to carbon support was 50/50. Catalyst loading for all
tests was ca. 1 mg cm.sup.-2. The Nafion-to-Catalyst ratio (NCR)
was 2 unless otherwise noted. 1 st Pyrolysis 2nd Pyrolysis Nominal
Catalytic Gas and Gas and Fe Content Activity (Temp.) (Temp.)
Pore-Filler (wt %) (A cm.sup.-3).sup.a NH.sub.3 (1050.degree. C.)
-- PTCDA 0.2 22 NH.sub.3 (1050.degree. C.) -- PTCDA 0.5 24 NH.sub.3
(1050.degree. C.) -- PTCDA 1.0 30 Ar (1050.degree. C.) -- Phen 0.2
2.8 Ar (1050.degree. C.) -- Phen 1.0 5.5 Ar (1050.degree. C.) --
Phen 4.1 1.4 Ar (1050.degree. C.) NH.sub.3 (950.degree. C.) Phen
0.2 60.sup.b Ar (1050.degree. C.) NH.sub.3 (950.degree. C.) Phen
1.0 64.sup.b Ar (1050.degree. C.) NH.sub.3 (950.degree. C.) Phen
4.1 31.sup.b Ar (1050.degree. C.) NH.sub.3 (950.degree. C.) Phen
1.0 99.sup.c (NCR 1.5) .sup.aConverted from the activity measured
under 1.5 bar O.sub.2 absolute pressure to the reference pressure
of 1 bar O.sub.2 absolute pressure (details available in the
Supplementary Information). .sup.bUnoptimized mass loss in 2.sup.nd
pyrolysis. .sup.cOptimized mass loss in 1.sup.st and 2.sup.nd
pyrolysis
[0204] FIG. 2 presents the polarization curves in terms of
volumetric current density for our best NPMC and for the presumed
best NPMC reported to date by Wood et al. (3M, 2008, REFERENCE 7).
The U.S. DOE volumetric activity target for ORR on NPMCs is
specified for 0.8V iR-free cell voltage. As shown in FIG. 2, the
kinetic activity (free of mass transport losses) of the NPMCs at
0.8 V iR-free cell voltage cannot be directly read from the
polarization curves, but must instead be estimated by extrapolating
the kinetically controlled Tafel slope observed at higher cell
voltage. Our best NPMC shows an activity enhancement of more that
25 times that of the previously highest NPMC activity.
[0205] To give prominence to the progress made by the NPMCs herein
for ORR in PEM fuel cells, FIG. 3 shows two polarization curves, in
terms of current density (Acm.sup.-2), of the best NPMC produced
herein (Table 1); one using a catalyst loading of 1.0 and the other
5.3 mgcm.sup.-2. These polarization curves are compared with that
of a Pt-based cathode catalyst (Gore PRIMEA.TM. 5510 MEA from W. L.
Gore & Associates, ca. 0.4 mgcm.sup.-2 Pt at cathode and anode)
tested under the same conditions and test fuel cell. It can be seen
in FIG. 3 that at 0.9 V iR-free cell voltage, within the
kinetically controlled Tafel region for both polarization curves,
increasing the loading of the NPMC by ca. 5 increases the current
density of the cell by about the same factor. It can also be seen
at 0.9V that the current density of the NPMC (5.3 mgcm.sup.-2) is
equal to that of the Pt-based catalyst. Although it may seem unfair
to compare a NPMC loading of ca. 5 mgcm.sup.-2 with a Pt loading of
0.4 mgcm.sup.-2, the limiting factor for the Pt loading is cost,
while no such factor exists for the low-cost NPMCs herein. However,
for current densities>0.1 Acm.sup.-2, the NPMC-based cathodes
display lower performance than the Pt cathode (FIG. 3). This is
probably caused by poor mass-transport properties that must be
improved.
[0206] Steps followed to obtain the data appearing in Tables 2 and
3. Four series of catalyst were prepared using PCTDA as the
pore-filler and Black Pearls 2000.TM. as the carbon support
corresponding to 4 different PTCDA nominal concentrations; 0, 25,
50 and 75%. The nominal Fe content for all catalysts was 0.2 wt %.
All pyrolyses were conducted either in Ar or in NH.sub.3 (as
specified in the Table captions) at 1050.degree. C.
[0207] Series 1. [0208] a. For each concentration of PCTDA several
catalysts were produced by varying the pyrolysis time, each time
resulting in a different mass loss. [0209] b. Fuel cell tests were
conducted for each catalyst in 1a. A Nafion-to-catalyst ratio of 2
was used for all tests. The kinetic currents at 0.8 V.sub.iR-free
was determined by first converting the current from Acm.sup.-2 to
Ag.sup.-1, then finding the intersection of the extrapolated Tafel
slope with the 0.8 V.sub.iR-free axis. An example of such kinetic
values vs. mass loss is found in FIG. 4 (hollow circles) for the
series of 10 catalysts prepared with 50% nominal PTCDA. The point
corresponding to the maximum catalytic activity at the optimal mass
loss is identified by an arrow in FIG. 4. This maximum activity (97
Ag.sup.-1) obtained in fuel cell testing at 0.8V.sub.iR free vs RHE
and 80.degree. C. is reported in Table 2 for 50% nominal PTCDA. The
polarization curve corresponding to this optimal catalyst for 50%
nominal PTCDA is depicted by the second line from the top when
looking at the left side of the graph in FIG. 5. [0210] c.
Micropore surface area measurements were performed for selected
catalysts in 1. An example of such micropore surface area values
vs. mass loss (corresponding to the 50% nominal PTCDA concentration
series) is found in FIG. 4 (hollow squares). Note how the maximum
kinetic current in this series coincides closely with the maximum
micropore surface area, suggesting that micropores play an
important role in the formation of catalytic sites.
[0211] A summary of the optimal polarization curves (in Acm.sup.-2)
corresponding to each of the four nominal PTCDA concentration
series described above is found in FIG. 5. All values of catalytic
activity (in Ag.sup.-1, obtained in fuel cell testing at 0.8
VR-free vs. RHE and 80.degree. C.), optimal mass loss, micropore
surface area and surface N content for the four catalysts shown in
FIG. 5 are listed in Table 2. Finally, 50% pore-filler
concentration was chosen to conduct the remainder of all
experiments herein.
TABLE-US-00002 TABLE 2 Catalytic activity (in A g.sup.-1, obtained
in fuel cell testing at 0.8 V.sub.IR-free vs RHE and 80.degree. C.,
under 1.5 bar O.sub.2), optimal mass loss during pyrolysis,
micropore surface area and N content for catalysts produced at
optimal mass loss with different nominal concentrations of PCTDA as
the pore-filler and a nominal Fe content of 0.2 wt %. Black Pearls
2000 .TM. was used as the carbon support. Pyrolysis was conducted
in NH.sub.3 at 1050.degree. C. Catalyst loading for all tests was
ca. 1 mg cm.sup.-2. The Nafion-to-Catalyst ratio was 2. The
volumetric activities under reference conditions of 1 bar O.sub.2
are obtained by multiplying the catalytic activities measured at
1.5 bar O.sub.2 (A g.sup.-1) by a factor of 0.231 as explained in
the following section. Nominal Catalytic Micropore PCTDA Activity
Optimal Mass Surface Area N Content (wt %) (A g.sup.-1) Loss (%)
(m.sup.2g.sup.-1 ) (at %) 0 8 30 741 -- 25 37 30 864 1.1 50 97 45
716 1.9 75 116 66 738 3.2
[0212] Series 2.
[0213] Three series of catalysts were produced using PCTDA as the
pore-filler and Black Pearls 2000.TM. as the carbon support with a
50% PTCDA nominal concentration, corresponding to three different
nominal Fe concentrations; 0.2 (already covered in 1.), 0.5 and 1
wt %. All pyrolyses were conducted in NH.sub.3 at 1050.degree. C.
[0214] a. For each nominal Fe concentration several catalysts were
produced by varying the pyrolysis time, each time resulting in a
different mass loss. [0215] b. Fuel cell tests were conducted for
each catalyst in 2a. The kinetic currents at 0.8 V.sub.iR-free were
determined as in 1b. The kinetic currents, in Ag.sup.-1 at
0.8V.sub.iR-free vs RHE and 80.degree. C. corresponding to the
optimal mass loss upon pyrolysis, are reported in Table 3. [0216]
c. Micropore surface area measurements were performed for the
optimal catalyst in each series.
[0217] All values of catalytic activity (in Ag.sup.-1, obtained in
fuel cell testing at 0.8 V.sub.iR-free vs RHE and 80.degree. C.),
optimal mass loss, micropore surface area and surface N content
corresponding to the optimal catalyst from each nominal Fe
concentration series are listed in Table 3 (rows 1-3).
TABLE-US-00003 TABLE 3 Catalytic activity (in A g.sup.-1, obtained
in fuel cell testing at 0.8 V.sub.iR-free vs RHE and 80.degree. C.,
under 1.5 bar O.sub.2), optimal mass loss during pyrolysis,
micropore surface area and N content for catalysts produced at
optimal mass loss using various combinations of pyrolysis steps,
pore-filler and nominal Fe content. Nominal Micropore Pyrolysis Fe
Catalytic Optimal Surface N Gas and Pore- Content Activity Mass
Area Content Temp. Filler (wt %) (A g.sup.-1) Loss (%) (m.sup.2
g.sup.-1) (at %) 1-Step NH.sub.3 PTCDA 0.2 97 45 716 1.9 Pyrolysis
(1050.degree. C.) 0.5 106 50 736 1.7 1.0 132 48 710 1.2 Ar Phen 0.2
12 26 188 2 (1050.degree. C.) 1.0 24 27 242 2.6 4.1 6 27 330 1.4
2-Step Ar + NH.sub.3 Phen 0.2 260 39* -- 2.2 Pyrolysis
(1050.degree. C.) (950.degree. C.) 1.0 277 35* 497 2.4 4.1 134 33*
392 2.4 Ar + NH.sub.3 Phen 1.0 429 47** 580 -- (1050.degree. C.)
(950.degree. C.) (NCR 1.5:1) Black Pearls 2000 .TM. was used as the
carbon support. The mass ratio of pore-filler to carbon support was
1. Catalyst loading for all tests was ca. 1 mg cm.sup.-2. The
Nafion-to-Catalyst ratio (NCR) was 2 unless otherwise noted.
*Combined mass loss from two pyrolyses (does not represent optimal
mass loss). **Combined mass loss from two pyrolyses (represents
optimal mass loss).
[0218] Series 3.
[0219] The same steps as in 2 were followed for three more series
of catalysts, this time produced using Phen as the pore-filler and
Black Pearls 2000.TM. as the carbon support with a 50% Phen nominal
concentration, corresponding to three different nominal Fe
concentrations; 0.2, 1 and 4.1 wt %. All pyrolyses were conducted
in Ar at 1050.degree. C. The results are summarized in Table 3
(rows 4-6).
[0220] Series 4.
[0221] Optimal catalysts from each series (0.2, 1 and 4.1 wt %
nominal Fe concentration) in 3. underwent a second pyrolysis, this
time in NH.sub.3 at 950.degree. C. for 5 minutes. The results are
summarized in Table 3 (rows 7-9).
[0222] The best catalyst from 4. (50% nominal Phen concentration
and 1.0 wt % nominal Fe concentration pyrolysed first in Ar at
1050.degree. C. then in NH.sub.3 at 950.degree. C.) was further
optimized by determining the optimal duration of the 2.sup.nd
pyrolysis, or mass loss (ca. 30%) and the optimal
Nafion-to-catalyst ratio (1.5). The results are summarized in Table
3 (row 10).
[0223] Conversion from the measured mass activity in Ag-1 under 1.5
bar O.sub.2 to volumetric activity in Acm-3 under reference
conditions of 1 bar O.sub.2. For comparison to DOE performance
targets, the activities of the NPMCs are reported in Acm.sup.-3 and
under US D.O.E. reference conditions of 1 bar O.sub.2 absolute
pressure, 100% RH and 80.degree. C. (REFERENCE 26). The
measurements were performed in reference conditions, except for the
O.sub.2 pressure (1.5 bar). To convert the mass activity (I.sub.M)
measured under 1.5 bar O.sub.2 absolute pressure to volumetric
activity (I.sub.V*) under the reference conditions of 1 bar O.sub.2
absolute pressure, Eq. I has been applied:
I V * = I M .rho. eff ( P O 2 * P O 2 ) 0.79 ( P H 2 * P H 2 )
.alpha. c / 2 Eq . 1 ##EQU00001##
where I.sub.M is the mass activity measured under the O.sub.2 and
H.sub.2 pressures P.sub.O2 and P.sub.H2 (1.5 bar each), and
P.sub.O2* and P.sub.H2 are the reference pressures (1 bar). A
correction must therefore be made to account for the difference in
O.sub.2 pressure (thermodynamic and kinetic correction) and H.sub.2
pressure (thermodynamic correction only) between reference and
actual test conditions. .rho..sub.eff is the effective density of a
carbon-based NPMC in the porous cathode. Since it has been shown
for Pt/C catalysts that the kinetic activity at a fixed cell
voltage is proportional to P.sub.O2.sup.0.79 and to
P.sub.H2.sup..alpha.c/2 (where .alpha..sub.c is the cathodic
transfer coefficient), the same law was assumed to apply to these
NPMC (REFERENCE 48). From FIG. 3, a Tafel slope of 61 mV/dec is
found for the most active NPMC in this study. This slope is nearly
equal to that of Pt/C and is similar to most other NPMCs herein.
The Tafel slope and .alpha..sub.c coefficient are related through
Eq. 2:
Slope Tafel = R T ln 10 .alpha. c F ( 2 ) ##EQU00002##
A value for .alpha..sub.c of 1.15 is obtained based on a slope of
61 mV/dec. Theoretically, .alpha..sub.c cannot exceed a value of 1.
However, assuming .alpha..sub.c=1 in Eq. 1, the value of I.sub.V*
would only increase by ca 3%. Next, .rho..sub.eff was previously
assumed to be 0.4 gcm.sup.-3 (REFERENCE 52). Our direct electrode
thickness measurements confirm that the value of .rho..sub.eff is
indeed very close to 0.4 gcm.sup.-3. This value has therefore been
retained. With regard to porosity, the effective density of
catalyst of 0.4 gcm.sup.-3 corresponds to ca 50% porosity for a
Nafion-to-NPMC ratio of 1.5. From Eq. 1, it can be seen that
multiplying the mass activity (in Ag.sup.-1; measured under
P.sub.O2 and P.sub.H2 of 1.5 bar, 100% RH and 80.degree. C.) by
0.230 yields the volumetric activity (Acm.sup.-3) under the
reference conditions (P.sub.O2 and P.sub.H2 1 bar, 100% RH,
80.degree. C.) defined by the U.S. DOE (REFERENCE 26).
[0224] In conclusion, the above demonstrates that sufficiently
active and inexpensive non-precious metal catalysts for ORR are
possible and therefore offer an alternative to Pt-based catalysts
and provide a path toward the commercial viability of PEMFCs. These
highly active and near-target-achieving iron-based electrocatalysts
have been realized thanks to an innovative synthesis method based,
in this case, on: (i) filling, by planetary ballmilling, the empty
pores of a highly microporous carbon support (Black Pearls
2000.TM.) with an N-bearing pore-filler (1,10 phenanthroline) and
an iron precursor (iron.sup.II acetate), and (ii) pyrolysing this
catalyst precursor first in Ar at 1050.degree. C., then in NH.sub.3
at 950.degree. C.
Example 2
[0225] Other catalysts according to the present have been prepared
by filling the micropores of a microporous support by planetary
ballmilling with the following pore-fillers: PTCDA,
##STR00002##
[0226] The loading of pore-filler ranged from 5 to 85 wt % based on
the total weight of the catalyst.
[0227] Using BlackPearl.TM. as the microporous support, these
catalysts showed satisfying catalytic activity for loading>50 wt
%.
[0228] Catalysts have also been prepared using different
microporous supports: BlackPearls.TM. (Cabot corp., BET area 1200
m.sup.2 g-1, surface micropore 900 m.sup.2 g.sup.-1), and
KetjenBlack.TM. (Akzo Nobel, BET area 1400 m.sup.2 g.sup.-1,
surface micropore 500 m.sup.2 g.sup.-1). Pyrolysis was performed in
Ar at temperatures ranging from 500 to 1100.degree. C. Interesting
catalytic activities have been observed at temperatures
T>800.degree. C.
[0229] Using PTCDA as a pore-filler, both KetjenBlack.TM. and
BlackPearls.TM. yielded catalysts with good activities.
[0230] Using KetjenBlack.TM. as a microporous support and PTCDA,
hexacarbonitrile or tetracarbonitrile as a pore-filler also yielded
catalysts with good activities.
Example 3
[0231] Catalysts similar to those of Example 1 were prepared with
iron acetate, ferrocene and ferricyanide as non-noble metal
precursors. These catalysts were prepared using 50 wt % of
phenanthroline and 50 wt % of BlackPearls.TM. with a nominal Fe
concentration of 0.2 wt %. Pyrolysis was performed under Ar or
NH.sub.3. The same good catalytic activities were obtained will all
non-noble metal precursors.
Example 4
[0232] This example investigates the influence, on the kinetic
activity of the catalyst, of (a) the type of pore-filler used, (b)
the mass ratio of pore-filler in catalyst precursor, (c) the type
of microporous carbon black used and (d) the nominal iron loading
in the catalyst precursor. This expands on the above, where two
types of pore-filler and one type of carbon black were used. Here,
four types of pore-filler and two types of carbon blacks were used
(see below). The pore-fillers used are (i)
perylene-tetracarboxylic-dianhydride (PTCDA, N-free) (REFERENCE
53), (ii) perylene tetracarboxylic-diimide (PTCDI, N-bearing),
(iii) 1,10 phenanthroline (phen, N-bearing) that complexes with
iron.sup.II and has a structure similar to a part of the catalytic
site (REFERENCE 31), and (iv) polyacrylonitrile (PAN, N-bearing).
The latter has already been used as a nitrogen precursor by
Yeager's group (REFERENCE 6). The microporous carbon blacks used
are (i) Black Pearls 2000 (BP) from Cabot (BET area 1379 m.sup.2
g.sup.-1, micropore area 934 m.sup.2 g.sup.-1), and (ii)
KetjenBlack EC600-JD (KB) from Azko-Nobel (BET area 1405 m.sup.2
g.sup.-1, micropore area 507 m.sup.2 g.sup.-1). The mass ratio of
pore-filler and the nominal iron loading used in the catalyst
precursors ranged from 0 to 90 wt % and 0.2 to 5 wt %,
respectively.
Experimental
[0233] Catalyst synthesis. Catalyst precursors were prepared using
planetary ballmilling. The iron precursor is iron.sup.II acetate
(FeAc). The molecular structures of the various pore-fillers used
are:
##STR00003##
[0234] Planetary ballmilling was performed using ca. 1 g of carbon
black and pore-filler mixture (the mass of each depending on the
targeted pore-filler mass ratio) and FeAc (the mass depending on
the targeted nominal iron loading in the catalyst precursor) was
placed in a hardened steel vial (ca. 65 cm.sup.3) together with 20
chrome-steel balls of 0.25 inch diameter. The ball-to-powder mass
ratio was 20:1. The vial was sealed in a nitrogen glove box. Once
tightly sealed, the vial was placed in a planetary ballmiller
(FRITSCH Pulverisette 7) to undergo 3 hours of ballmilling at 400
rpm. The resulting powder formed the catalyst precursor. When phen
was used as the pore-filler, the phen and iron.sup.II (from FeAc)
were first complexed by mixing both in a solution of ethanol to
form a [Fe(Phen).sub.3].sup.2+ complex. The carbon black was then
added to the solution. This solution was stirred over low heat on a
magnetic hotplate for 2 hours and then placed in a drying oven
overnight to be completely dried in air at 90.degree. C. Once
dried, the powder was placed in a steel vial and underwent the same
ballmilling processing steps as described above.
[0235] Two heat-treatment methods for the catalyst precursors were
used to obtain active ORR catalysts. In method I, the catalyst
precursor was pyrolysed in ammonia at 1050.degree. C. A number of
catalysts, each having a different mass loss were obtained by
pyrolysing samples of the same catalyst precursor for various
pyrolysis times. In Method II, the catalyst precursor was first
pyrolysed in Ar at 1050.degree. C. for 60 minutes. Then, the
resulting powder was pyrolysed in NH.sub.3 at 950.degree. C. Here
again, for the pyrolysis in NH.sub.3, a number of catalysts, each
having a different mass loss was obtained by pyrolysing samples of
the same catalyst precursor for various pyrolysis times. The
pyrolysis procedure is described in more detail above.
[0236] Electrochemical analysis. The catalyst ink for the fuel cell
cathode was prepared with a Nafion-to-catalyst mass ratio (NCR) of
2 for all samples except for catalysts made with phen where the NCR
value was 1.5. Details of the ink preparation and MEA assemblies
may be found above. The fuel cell tests were performed at
80.degree. C. cell temperature, with the humidifier for H.sub.2 and
O.sub.2 at 105.degree. C. and 95.degree. C. respectively. The back
pressures were 15 psig on both sides. The electrode geometric area
was 1.14 cm.sup.2. During the initial fuel cell warm-up period,
nitrogen was fed to both electrodes. When the fuel cell reached
80.degree. C., nitrogen was switched to hydrogen and oxygen at
anode and cathode, respectively. The system was then held at OCV
for 15 minutes before making an impedance measurement and recording
the first polarization curve at a potential scan rate of 0.5 mV/sec
using a Parstat 2273 potentiostat. The kinetic current at 0.8 V
iR-free was estimated by extrapolating the Tafel slopes observed at
higher potential. Conversion from the measured mass activity in A
g.sup.-1 at 1.5 bar O.sub.2 and 1.5 bar H.sub.2 to volumetric
activity in A cm.sup.-3 at reference conditions of 1 bar is
explained in detail above.
[0237] Physical analysis. Surface area measurement was performed
with a Quantachrome Instruments Autosorb-1 and with N.sub.2 as the
adsorbate. Isotherm analysis was performed as in REFERENCE 19.
Surface elemental analysis was performed via X-ray photoelectron
spectroscopy using a VG Escalab 200i instrument and the
monochromatic source was the Al K.alpha. line (1486.6 eV).
Quantification of the elements was performed using Casa XPS
software.
RESULTS AND DISCUSSION
[0238] One-step pyrolysis (method I). The planetary ballmilling
method is an effective method for filling the pores of a carbon
black. While pristine Black Pearls 2000 (BP) has a micropore
surface area of 934 m.sup.2 g.sup.-1, after ballmilling of a
mixture of 50% BP and 50% PTCDA, the corresponding catalyst
precursor had a micropore surface area of only 65 m.sup.2 g.sup.-1
(FIG. 6).
[0239] The results shown in FIG. 6 were obtained for samples
prepared with BP and PTCDA, with a PTCDA/BP mass ratio of 50/50 and
1 wt % iron nominal loading as FeAc. The mass loss is used as the
x-axis variable in FIG. 6. Both the kinetic activity and the
micropore surface area reach a maximum at a mass loss of 50% during
the NH.sub.3 pyrolysis. FIG. 6 shows that the activity correlates
with the micropore surface area of the catalysts. Moreover, the
maximum microporous surface area is ca. 700 m.sup.2 g.sup.-1, i.e.
ca. 75% of the original micropore surface area found in pristine
BP. The activity does not, however, correlate with the N content in
the catalysts. Also, note that 50% mass loss corresponds to the
mass ratio of pore-filler in the catalyst precursor. For other
ratios of pore-filler/carbon black it was found that the maximum of
activity also occurred at a mass loss corresponding to that of the
mass ratio of the pore filler in the catalyst precursor. This is
understandable since (i) micropores control the activity of such
catalysts and (ii) the micropores arise from the etching by
NH.sub.3 of the pore-filler in the pores of the microporous carbon
black.
[0240] For the remainder of this study, for each carbon
black/pore-filler/Fe nominal loading combination, only the
optimized catalyst is reported, i.e. the catalyst for which the
mass loss in NH.sub.3-pyrolysis is about equal to the pore-filler
mass ratio in the catalyst precursor.
[0241] FIG. 7A shows the effect of the PTCDA loading in the
catalyst precursor on three characteristics of the resulting
catalysts: (i) the nitrogen content (ii) the micropore surface area
and (iii) the kinetic current in a PEM fuel cell. These catalysts
were prepared using method I and the nominal iron loading in the
catalyst precursors was fixed at 0.2 wt %. The ORR activity in fuel
cell appears to follow the same trend as nitrogen content with
PTCDA loading, except for a PTCDA loading of 90%, where we observe
a sudden decrease in nitrogen content, but no decrease in activity.
The micropore surface area is roughly equal for all PTCDA loadings
and corresponds in part to micropores created during pyrolysis due
to the etching of pore-filler by ammonia. These observations might
lead to the conclusion that the N content limits the activity.
However, an alternative explanation is also possible. Although the
nominal iron loading was fixed at 0.2 wt %, the final iron content
in the catalysts of FIG. 7A changes: it may be estimated as 0.2 wt
%100/(100-X), where X is the PTCDA mass ratio in wt %. This
calculation is based on the assumption that all iron content
present in the catalyst precursor remains in the catalyst, while
the PTCDA content is etched by NH.sub.3. The retention of Fe during
pyrolysis has been shown for a series of prior art catalysts
(REFERENCE 21). For example, the catalyst in FIG. 7A made using 75
wt % PTCDA in the catalyst precursor contains ca. 0.8 wt % Fe while
the one made with 20 wt % PTCDA contains ca. 0.25 wt % Fe. An
increase of catalytic activity by increasing iron loading up to 1
wt % Fe will be shown below when discussing FIG. 7B. The activity
of the series of catalysts in FIG. 7A can therefore be limited by
either the N content or the Fe content or a combination of
both.
[0242] In conclusion, for a nominal iron loading of 0.2 wt % (FIG.
7A), the highest kinetic activity was 116 Ag.sup.-1 at 0.8V iR-free
and was obtained for a PTCDA loading of 75%. The optimal mass loss
for this catalyst was 75%, i.e. the mass ratio of PTCDA in the
catalyst precursor.
[0243] Next, with a fixed PTCDA mass ratio of 50% in the catalyst
precursor, the nominal iron loading was varied from 0.2 to 5 wt %.
For all nominal iron loadings, pyrolysis in NH.sub.3 at
1050.degree. C. were repeated until ca. 50 wt % mass loss was
obtained; a value corresponding to the mass ratio of PTCDA in the
catalyst precursor. Consequently, the micropore surface area for
all these catalysts (see FIG. 7B, upper graph, stars) are almost
exactly the same. The time of pyrolysis required to obtain 50% mass
loss increased with increasing nominal iron loading. This is
believed to be due to a decreasing reaction rate between PTCDA and
NH.sub.3 with increasing iron content (REFERENCE 54), and may be
attributed to the competing decomposition reaction of NH.sub.3 into
N.sub.2 and H.sub.2 in the presence of excess iron (in the form of
aggregates), as it is known to be an effective catalysts for such a
reaction (REFERENCE 55).
[0244] The upper graph in FIG. 7B shows that the nitrogen content
in the catalysts (square) decreases with nominal iron loading up to
ca. 1 wt % and remains constant beyond this value. The decrease in
N content with increase in nominal iron loading may be a result of
the competing decomposition reaction of NH.sub.3 described above.
The ORR kinetic activity in fuel cell is shown in the lower graph
in FIG. 7B. It increases with increasing iron loading up to 1 wt %,
then decreases gradually with increasing nominal iron loading
beyond 1 wt %.
[0245] Thus, the kinetic activity of catalysts with increasing
nominal iron loading up to 1 wt % appears to be limited by iron
content, while the low kinetic activity of catalysts with nominal
iron loadings>1 wt % seem to be limited by low N content.
[0246] For the catalysts shown in FIG. 7B, the maximum kinetic
activity obtained was 132 Ag.sup.-1 at 0.8V iR-free and corresponds
to a nominal Fe loading of 1 wt %. This maximum kinetic activity is
believed to be the result of an optimal balance between Fe and N
content.
[0247] Two-steps pyrolysis (method II). The results obtained with
method II (first pyrolysis in Ar, second pyrolysis in NH.sub.3) are
shown in FIG. 8. The pore-filler used in this case is phen, the
carbon black and iron precursor are still BP and FeAc,
respectively. FIG. 8 is laid out similarly to FIG. 7, except that
phen is used as the pore-filler and method II was used for the
pyrolysis steps (see experimental).
[0248] FIG. 8A shows how N content, microporous surface area and
ORR kinetic activity in fuel cell varies with the phen mass ratio
in the catalyst precursor. The iron loading was fixed at 1 wt %. As
the phen mass ratio increases, the micropore surface area
continually decreases while the nitrogen content initially
increases (up to ca. 50 wt % phen) then levels-off. Unlike the
catalysts presented in FIG. 7, where the sole N precursor is
NH.sub.3, here two N precursors are introduced, namely phen and
NH.sub.3. The ORR activity in fuel cell seems to be influenced by
both the N content and the micropore surface area. The maximum
kinetic activity for catalysts made with phen as the pore-filler
was 429 Ag.sup.-1 at 0.8V iR-free using 50 wt % phen mass ratio
(FIG. 8A, lower graph). The iron content of this catalyst is ca. 2
wt % (based on 1 wt % nominal iron loading and ca. 50 wt % mass
loss during pyrolysis).
[0249] Next, the phen mass ratio was fixed at 50 wt % and the iron
loading was varied from 0.2 to 5 wt % (FIG. 8B). The micropore
surface area was roughly the same for different nominal iron
loadings, but the N content behaves differently with a maximum of
ca. 2.4 at % for a Fe loading of 1 wt %. The kinetic activity for
these catalysts follows the same trend as the N content. If one
were to plot the activity vs. N content, a linear relation would be
observed.
[0250] Overall, for all catalysts presented in FIG. 8, the optimum
Fe loading was 1 wt % for a 50 wt % phen mass ratio corresponding
to a maximum kinetic activity of 429 Ag.sup.-1 at 0.8V iR-free.
This activity is higher than the maximum activity for catalysts
presented in FIG. 7 (132 Ag.sup.-1 at 0.8V iR-free).
[0251] Effect of pore-filler used with method I (one-step
pyrolysis). Catalysts made with three different pore-fillers
(PTCDA, PTCDI and PAN) and heat treated in NH.sub.3 at 1050.degree.
C. were investigated. The pore-filler mass ratio was kept constant
at 50 wt % and the nominal iron loading was fixed at 1 wt %.
Pyrolysis were repeated until 50 wt % mass loss was obtained for
catalysts made with each pore-filler.
[0252] The first three rows in Table 4 represent the results for
catalysts made with PTCDA, PTCDI and PAN, respectively. The
nitrogen content and microporous surface areas of these catalysts
are similar. The kinetic activity for the catalyst made with PTCDA
is about twice as high as that made with PTCDI or PAN. For the
latter, the apparent Tafel slope is higher than that of either the
PTCDA- or the PTCDI-based catalyst (inset of FIG. 9). This results
in a low kinetic activity at 0.8V iR-free for the PAN-based
catalyst (Table 4). Thus, the comparison of the kinetic activities
should preferably be done at 0.9V rather than at 0.8V.
[0253] Effect of microporous carbon black used with method I
(one-step pyrolysis). Catalysts made with two different carbon
blacks (BP and KB), but similar methods were investigated. The
pore-filler and mass ratio used for both catalysts was PTCDA and 50
wt %, respectively. The iron loading was 1 wt %. The catalyst
precursor prepared by planetary ballmilling was pyrolysed in
NH.sub.3 at 1050.degree. C. Pyrolysis were repeated until 50 wt %
mass loss was obtained for both catalysts. Rows 1 and 4 in Table 4
represent these two catalysts, where the only difference is the
microporous carbon black used. The kinetic activity obtained with
the catalyst using KB+PTCDA is lower than that obtained with
BP+PTCDA, all other things equal. This lower kinetic activity is
consistent with the lower microporous surface area and N content in
the (KB+PTCDA)-based catalyst.
[0254] Activity and mass-transport: a necessary trade-off? While
high kinetic activity at 0.8 or 0.9V iR-free is desirable, so is
high power density at reasonably high cell voltage. Thus, good mass
transport properties of electrodes made with these catalysts is
also desirable. FIG. 9 presents the fuel cell polarization curves
recorded using O.sub.2/H.sub.2 for all catalysts listed in Table 4.
The catalyst loading was 1 mgcm.sup.-2. All catalysts were
synthesized according to method I (one pyrolysis in NH.sub.3 at
1050.degree. C.) except for the catalyst BP+phen which was
synthesized according to method II (first pyrolysis in Ar and the
second in NH.sub.3). FIG. 9 shows that the performance in fuel cell
at 0.5 V iR-free does not necessarily correlate with the kinetic
activity of the catalysts, measured at 0.8 or 0.9 V iR-free. The
higher kinetic activity of the BP+phen catalyst does not translate
into better performance at 0.5V. The KB+PTCDA catalyst, for
example, has much lower activity at 0.8V or 0.9V iR-free, but
better performance than the BP+phen catalyst at 0.5V iR-free.
TABLE-US-00004 TABLE 4 Nitrogen content, microporous surface area,
and kinetic activity at 0.9 V (in A g.sup.-1) and 0.8 V iR-free (in
A g.sup.-1 and A cm.sup.-3), for catalysts made with various
combinations of carbon black/pore-filler. In all cases the
pore-filler mass ratio and nominal iron content was 50% and 1 wt %,
respectively. Current Current Current Catalyst Nitrogen Micropores
at 0.9 V at 0.8 V at 0.8 V precursor at % m.sup.2 g.sup.-1 A
g.sup.-1 A g.sup.-1 A cm.sup.-3 BP + PTCDA 1.2 710 2.6 132 30 BP +
PTCDI 0.7 802 1.2 76 17 BP + PAN 1.1 821 1.2 36 8 KB + PTCDA 0.6
582 0.9 37 8 BP + Phen 2.4 580 6.8 429 99
[0255] Although the present invention has been described
hereinabove by way of specific embodiments thereof, it can be
modified, without departing from the spirit and nature of the
subject invention as defined in the appended claims.
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References